Bulugh al-Maram

Lexical Explanation:
«وِكَآءُ السَّهِ اَلْوِكَآءُ» The "waw" has a kasrah underneath, and the "alif" has a madd. It refers to the thread or rope with which the mouth of a waterskin, etc., is tied.
«اَلسَّهِ» The "sin" has a fathah, and the "ha" is shortened. It is used in the meaning of "dubr" (backside).
«اِسْتَطْلَقَ» To become open, to become loose.
«مُضْطَجعًا» To lie down on one's side.

Benefits and Issues:
➊ From the mentioned hadith, it is understood that sleep in itself does not invalidate ablution (wudu), but it creates a suspicion or likelihood of ablution being nullified. However, there is weakness in the chains of both narrations because among their narrators is one named "Baqiyyah," about whom many hadith scholars have said that his narrations are not clear (authentic). However, this weakness is slight; nevertheless, Mundhiri, Nawawi, and Ibn al-Salah have declared the hadith of Sayyiduna Ali radi Allahu anhu to be hasan (good).
➋ The hadith states that ablution is nullified in the state of sleeping while lying down, and in another narration, it is stated that ablution is nullified by sleep in general. The reconciliation between both hadiths is that deep sleep usually occurs when lying on one's side. In such a state, the limbs of the body become relaxed. In this situation, it is most likely that wind (passing gas) may be released, whereas this is not the case in light sleep.
➌ This does not mean that if one sleeps deeply while lying flat on the back or stomach, ablution is not nullified. Deep sleep, in whatever position it occurs, will invalidate ablution. It is just that deep sleep usually happens when lying on the side, hence its specific mention.

Narrator of the Hadith:
(Hadrat Muawiyah radi Allahu anhu) By Muawiyah is meant Hadrat Muawiyah ibn Abi Sufyan ibn Harb radi Allahu anhuma. On the occasion of the conquest of Makkah, both father and son accepted Islam. After the death of his brother Yazid ibn Abi Sufyan, Hadrat Umar radi Allahu anhu appointed him as the governor of Syria. He remained in this governorship until the caliphate of Hadrat Ali radi Allahu anhu. After Hadrat Hasan radi Allahu anhu announced his abdication from the caliphate, allegiance was given to him, and by consensus, he was appointed as the Amir. This event occurred in 40 AH. He passed away in the month of Rajab, 60 AH. At that time, he was 78 years old.

English Commentary:
1:
In the case of lying flat on the back, the likelihood of wind being released increases, whereas in light sleep this is not the case. Although this hadith is weak in terms of its chain of narration, the portion regarding "ablution (wudu) being nullified by sleeping while lying down" is supported by other narrations.

Note:
(Abu Khalid Yazid bin Abdur Rahman Darimi Dalani is a mudallis (one who practices tadlis in hadith transmission). He was often afflicted with much doubt. Shu’bah says that Qatadah did not hear this hadith from Abu Al-‘Aliyah, meaning there is also a break (inqita’) in the chain.)

Benefits and Issues:
➊ In summary, the hadith "Ablution (wudu) is required only for the one who sleeps lying down" is weak in its chain of narration, but its meaning and ruling are correct.
➋ It was a special characteristic of the Messenger of Allah (sallallahu alayhi wa sallam) that his heart remained awake during sleep; therefore, if his ablution (wudu) had been invalidated, he would have known.
➌ The summary of the four hadiths that Qatadah heard from Abu’l-‘Aliyah is as follows:
(First) It is not befitting for any servant to say that I (i.e., Muhammad sallallahu alayhi wa sallam) am superior to Yunus ibn Matta. [سنن ابي داود‘حديث : 4669]
(Second) The hadith of Ibn ‘Umar: The Messenger of Allah (sallallahu alayhi wa sallam) forbade performing any prayer after the dawn (fajr) prayer until the sun rises, and likewise after the afternoon (‘asr) prayer until the sun sets. [صحيح بخاري‘حديث : 585]
(Third) Judges are of three types: one will go to Paradise and two will go to Hell. The one who will go to Paradise is he who recognized the truth and judged accordingly. The second is he who recognized the truth but acted unjustly in his judgment; he is of the people of Hell. The third is he who judges out of ignorance; he too is of the people of Hell. [سنن ابي داود‘ حديث : 3573]
(Fourth) The hadith of Ibn ‘Abbas: The Messenger of Allah (sallallahu alayhi wa sallam) forbade prayer after the dawn (fajr) prayer until the sun rises, and also after the afternoon (‘asr) prayer until the sun sets. [صحيح بخاري حديث : 581]
Among these four hadiths, the hadith mentioned in this chapter is not included; therefore, its hearing (sama‘) is questionable.