Nasai

1535. Commentary: Its method will be as follows: the followers (muqtadis) will stand in two rows and begin the prayer behind the imam at the same time. However, when the imam performs bowing (ruku‘) and prostration (sujud), only the front row will perform bowing and prostration with the imam, while the back row will remain standing and keep watch over the enemy. In an armed state, they should be ready to respond to any attack from the enemy. When the front row completes the bowing and prostration of the first unit (rak‘ah), they will move to the back, and the back row will come forward. Now, these (who have come forward) will perform bowing and prostration with the imam in the second unit (rak‘ah), while the new back row will remain standing and provide protection. Then, both rows will conclude the prayer with the imam by saying salam. In this manner, both groups will have performed the prayer at the same time and also continued to protect one another.

Hadith Commentary:
According to Hafiz Ibn Hajar rahimahullah, Imam Bukhari rahimahullah has described another form of the prayer of fear (salat al-khawf). It is his habit that sometimes, as a matter of scholarly subtlety, he establishes a heading, but his real objective is to present the narration.

At this point, the second form of the prayer of fear is that if the enemy is in the direction of the qiblah, then the combatants will not be divided into two separate groups. However, if the enemy is in a direction other than the qiblah, then they will be divided into two separate groups, as has been clarified earlier in the narration reported from Ibn Umar radi Allahu anhu.

According to this narration, all the combatants will begin the prayer with the imam; then, only those in the row directly behind the imam will perform the bowing (ruku) and prostration (sujud), while those in the second row will remain engaged in guarding them. When the imam completes one rak‘ah, the first row will move back, as is explicitly mentioned in one narration (Sunan al-Nasa’i, Salat al-Khawf, Hadith: 1535). Then, the second row will come forward and perform the second rak‘ah with the imam; none of them will make up any missed rak‘ah, as is clarified in other ahadith (Sunan al-Nasa’i, Salat al-Khawf, Hadith: 1534).

In a narration reported from Hudhayfah radi Allahu anhu, it is also explicitly stated that they did not perform the second rak‘ah. In this form, the imam will perform two rak‘ahs, and the combatants will perform one rak‘ah each. This form of the prayer of fear is also described in the hadith reported from Jabir ibn Abdullah radi Allahu anhu (Sunan al-Nasa’i, Salat al-Khawf, Hadith: 1548).

This form is also supported by another narration reported by Ibn Abbas radi Allahu anhu, in which he stated that Allah, through your Messenger, has made four rak‘ahs obligatory in residence, two rak‘ahs in travel, and one rak‘ah in the prayer of fear (Sahih Muslim, Salat al-Musafirin, Hadith: 1575 (687)).

According to the majority, it is not permissible to suffice with only one rak‘ah in the prayer of fear, but in view of the explicit and authentic ahadith, their position is questionable.

And Allah knows best.
(Fath al-Bari: 2/558)

1534. Commentary: See Benefit Number 3 of Hadith Number 1531.

1536. Commentary: This too is one of the methods of Salat al-Khawf (the Prayer of Fear).