Bukhari

Hadith Commentary:
In this narration, only the performance of the Zuhr prayer is mentioned in the case of commencing a journey after the sun has passed its zenith (zawal al-shams). This means that the way of combining two prayers here is that the first prayer is performed in the time of the second prayer, which is termed in technical language as "delayed combination" (jam‘ ta’khir). This also appears to be the inclination of Imam Bukhari rahimahullah. However, when Imam Ishaq ibn Rahwayh narrated this hadith, his wording is as follows: If the Messenger of Allah sallallahu alayhi wa sallam was on a journey and the sun had passed its zenith, he would combine Zuhr and Asr, and then begin the journey. This narration implies that "advanced combination" (jam‘ taqdim) is also permissible.

This is further supported by another narration which Imam Hakim rahimahullah has mentioned in his "Arba‘in," that if the sun had passed its zenith before setting out, the Prophet sallallahu alayhi wa sallam would combine Zuhr and Asr and then mount (his ride). This can be presented as corroboration (mutaba‘at) for the narration of Ishaq ibn Rahwayh. (Fath al-Bari: 2/753)

The narration of Mu‘adh ibn Jabal radi Allahu anhu is also presented as evidence for advanced combination (jam‘ taqdim), that when the Messenger of Allah sallallahu alayhi wa sallam intended to travel after the sun had passed its zenith, he would perform Zuhr and Asr together. (Musnad Ahmad: 5/241)

The Messenger of Allah sallallahu alayhi wa sallam did this on the occasion of the Battle of Tabuk. (Al-Sunan al-Kubra lil-Bayhaqi: 3/163)

A similar narration is also reported from Anas ibn Malik radi Allahu anhu. (Al-Sunan al-Kubra lil-Bayhaqi: 3/164)

A similar narration is also reported from Ibn Abbas radi Allahu anhu. (Al-Sunan al-Kubra lil-Bayhaqi: 3/164)

It is narrated from Ibn Abbas radi Allahu anhu that when the Messenger of Allah sallallahu alayhi wa sallam encamped at a place he liked, he would combine Zuhr and Asr, then begin the journey; and if a suitable place was not available, he would continue the journey, then halt at a suitable place and combine Zuhr and Asr there. (Al-Sunan al-Kubra lil-Bayhaqi: 3/163)

All its narrators are trustworthy, but there is doubt regarding its being marfu‘ (attributed directly to the Prophet sallallahu alayhi wa sallam). However, Imam Bayhaqi rahimahullah has also narrated this report with strong certainty as mawquf (attributed to the Companion).

It should be noted that besides combining prayers during travel, it is also permissible to combine them due to rain, illness, or any other emergency need. (Al-Sunan al-Kubra lil-Bayhaqi: 3/169)

In summary, it is permissible to combine prayers in cases of unavoidable circumstances and severe necessity, such as travel, fear, illness, rain, severe windstorm, or any emergency need. However, without severe necessity, it is not permissible to combine two prayers, as is the common practice among some businessmen who do so out of laziness or business preoccupation. This is not correct; rather, it is a grave sin.

It is narrated from some of the noble Companions radi Allahu anhum that combining prayers without excuse is a major sin. (Fath al-Bari: 2/753)

In this regard, a narration from Ibn Abbas radi Allahu anhu is presented, that he once combined prayers without an excuse, although the excuse mentioned in the narration is clearly stated. (Sahih Muslim, Salat al-Musafirin, Hadith: 1636(705))

Hadith Commentary:
1.
Allamah Kirmani rahimahullah states:
The application of the word "salat" (prayer) indicates that the Messenger of Allah sallallahu alayhi wa sallam would perform these prayers with their pillars (arkan), conditions (shurut), and recommended acts (sunan), etc., and that the adhan (call to prayer) and iqamah (second call) are considered sunnah for the prayer. Therefore, he would also make arrangements for the adhan and iqamah. When something is mentioned in an absolute sense, what is meant is its complete form.
(Fath al-Bari: 2/750)

(2)
According to us, Imam Bukhari rahimahullah has explained the hadith narrated from Anas radi Allahu anhu by means of the hadith narrated from Ibn Umar radi Allahu anhu, because in the hadith of Anas radi Allahu anhu there is no mention of adhan or iqamah, whereas in the narration from Ibn Umar radi Allahu anhu, it is explicitly stated that iqamah was given for Maghrib and Isha. Thus, in the narration of Sunan al-Daraqutni, it is mentioned that Umar radi Allahu anhu, while traveling, would not give the adhan but, after giving the iqamah, would combine the two prayers, that is, he would give a separate iqamah for each prayer.
(Fath al-Bari: 2/750)

(3)
This hadith does not clarify to what extent the Maghrib prayer should be delayed.
In one narration, it is mentioned that it would be delayed until the redness (after sunset) disappeared, then after stopping (at the campsite), Maghrib and Isha would be combined and performed.
(Sahih al-Bukhari, al-Jihad wal-Siyar, Hadith: 3000)
In a lengthy narration from Jabir radi Allahu anhu, it is mentioned that the Messenger of Allah sallallahu alayhi wa sallam, at Muzdalifah, combined the Maghrib and Isha prayers with one adhan and two iqamahs.
(Sahih Muslim, al-Hajj, Hadith: 2950(1218))
The preferred opinion is that there should be a separate iqamah for each prayer, but one adhan will suffice.

Hadith Commentary:
In one narration, it is mentioned that the Dhuhr prayer would be delayed to such an extent that the beginning time of the ‘Asr prayer would commence, then they would halt and combine the Dhuhr and ‘Asr prayers and perform them together.
In another narration, it is mentioned that during travel, the Maghrib prayer would also be delayed, then after the disappearance of twilight, the Maghrib and ‘Isha prayers would be combined and performed together. In any case, Imam Bukhari is of the opinion of combining Dhuhr and ‘Asr by delaying (jama‘ ta’khir).
Imam Bukhari rahimahullah’s stance regarding combining prayers by advancing (jama‘ taqdim) is not clear.
(Fath al-Bari: 2/752)

Hadith Commentary:
Benefits and Issues:
If there was no need for haste, the Prophet sallallahu alayhi wa sallam would perform the Zuhr (noon) prayer and then set out on the journey, and would perform the Asr (afternoon) prayer at its appointed time.
If swiftness was required, then after the sun had passed its zenith, he would combine the Asr prayer with the Zuhr prayer, just as the Prophet sallallahu alayhi wa sallam did during the journey of the Battle of Tabuk.

1204. Commentary:
➊ Knowledge of the prayer times and the actual arrival of its time are among the important conditions for the validity of prayer, and in this regard, the imam and the muezzin are responsible; the doubt of any single individual is not to be considered. The doubt expressed by Anas radi Allahu anhu is, in reality, merely a doubt, because the Prophet sallallahu alayhi wa sallam never performed the Zuhr prayer before the sun had passed its zenith. Therefore, the followers should place their trust in their imam.
➋ This also shows that the Prophet sallallahu alayhi wa sallam used to perform the prayer at the very beginning of its time as soon as the sun declined, and he would give importance to this even while traveling.

1218. Commentary:
Some people deduce from this hadith that "early combining" (jam‘ taqdim) is not valid (i.e., that the ‘Asr prayer should not be performed at the time of Zuhr). However, "early combining" (jam‘ taqdim) is established by several other authentic ahadith, such as the previous hadith of Mu‘adh radi Allahu anhu in Sunan Abi Dawud, hadith [1208]. There is no harm in interpreting these various ahadith as applying to different circumstances.

595. Commentary: In this narration as well, there is combining of prayers by delaying (jama‘ ta’khir).

587. Commentary:

➊ If a traveler wishes to combine Dhuhr and Asr, and likewise Maghrib and Isha, then there are three forms of this: The first is "jam‘ taqdim" (combining by advancing), that is, to combine Asr with Dhuhr or Isha with Maghrib at the time of the earlier prayer. The second form is "jam‘ ta’khir" (combining by delaying), that is, to delay Dhuhr and perform it with Asr, or delay Maghrib and perform it with Isha at the time of the later prayer. The third form is "jam‘ suri" (apparent combining), that is, to perform Dhuhr at its last time and Asr at its earliest time, and similarly, to perform Maghrib at its last time and Isha at its earliest time. All three forms are permissible, because all of these forms are established from the Prophet sallallahu alayhi wa sallam. Therefore, to deny the first two forms and insist only on "jam‘ suri" abolishes the ease (yusr) that has been granted by the Shari‘ah.

➋ Regarding the mentioned hadith, some people say that it refers to "jam‘ suri," but the wording of the hadith does not support this. It mentions performing Dhuhr at the time of Asr, so its clear meaning is that you performed "jam‘ ta’khir," that is, you performed Dhuhr at the time of Asr along with Asr. And even more explicitly, the wording comes: «أخر الظهر الی أن یدخل وقت العصر» "The Prophet sallallahu alayhi wa sallam delayed Dhuhr to such an extent that the time of Asr entered." See: [صحیح مسلم ، صلاة المسافرین ، حدیث : 704].

Similarly, in other narrations it is mentioned that when you would begin a journey after the sun had passed its zenith, you would also perform Asr with Dhuhr (i.e., you would perform "jam‘ taqdim"). This hadith is concise, whose clarification is found in other ahadith. In the following ahadith, all these forms will be mentioned. However, during Hajj at Arafat, performing Dhuhr and Asr at the time of Dhuhr is agreed upon, and this is "jam‘ taqdim." [صحیح مسلم ، الحج ، حدیث : 1218]

Takhrij:
«أخرجه البخاري، تقصير الصلاة، باب يؤخر الظهر إلي العصر إذا ارتحل قبل أن تزيغ الشمس، حديث: 1111، ومسلم، صلاة المسافرين، باب جواز الجمع بين الصلاتين في السفر، حديث:704، وأبو نعيم في المستخرج:2 /294.»©Explanation:
According to this hadith, it is proven permissible to combine the Zuhr and Asr prayers, as well as the Maghrib and Isha prayers, during travel.
In this, both combining by advancing (jama‘ taqdim) or delaying (jama‘ ta’khir) are established.
The Hanafis do not accept actual combining (jama‘ haqiqi); they accept only apparent combining (jama‘ suri).
However, from the hadith in Jami‘ al-Tirmidhi, it is established that this was not apparent combining but actual combining. Its meaning is that when you (sallallahu alayhi wa sallam) would set out on a journey before the sun had declined, you would delay Zuhr and combine it with Asr, performing both together. And when you would begin the journey after the sun had declined, you would combine Asr with Zuhr and perform both together.
(Jami‘ al-Tirmidhi, al-Jumu‘ah, Hadith: 553) Imam Tirmidhi has graded this narration as hasan.
And the hadith from Mustakhraj Abu Nu‘aym also supports this, which the author rahimahullah has mentioned.
And this is the preferred view.
And Allah knows best.