Imām Muḥammad ibn Ṣāliḥ al-ʿUthaymīn (d. 1421 AH) said:

The scholars have differed concerning whether sleep itself is considered a nullifier of wuḍūʾ, or if it is the presumptive place for other nullifiers [while not being a nullifier itself]. The opinions are:

1. Sleep Nullifies Wuḍūʾ

Whether it be slight drowsiness or deep sleep, however the sleep has occurred [i.e., whether sitting or lying down]. The evidence of this is the ḥadīth of Ṣafwān: “The Messenger of Allāh (صلى الله عليه وسلم) used to order us when travelling to not remove our socks for three days and two nights except if we we were in a state of janābah [major impurity that would obligate ghusl]. However, [wiping over our socks for wuḍūʾ was made permissible] in the case of urination, defecation, and sleep.”¹

2. Sleep Does Not Nullify Wuḍūʾ

The evidence of this is the ḥadīth of Anas (رضي الله عنه) that the ṣaḥābah—may Allāh be pleased with all of them—used to wait for ʿIshāʾ prayer for so long during the time of the Messenger of Allāh (صلى الله عليه وسلم), to the extent that they would nod off; their heads dropping in drowsiness. Then, they would pray without performing wuḍūʾ². In the narration of al-Bazzār: “They would lie down on their sides.”³

The Opinion of the Ḥanbalī Madh`hab: Sleep Itself Is Not a Nullifier of Wuḍūʿ

Rather, sleep is a state where clear nullifiers of wūḍu ‘may’ occur. Thus, sleep of any kind nullifies except of the type of sleep in which a nullifier⁴ is very unlikely to occur; in which one would not expect to lose his conciseness completely. The [Ḥanbalī] author concludes: “Fading of the mind (i.e. loss of awareness) is a nullifier of wuḍūʾ, except for light drowsiness such that may occur from a (conscious) standing or sitting person.’

The Stance of Shaykh al-Islām Ibn Taymiyyah

Sleep is a presumptive place for the occurrence of a nullifier (of wuḍūʿ). If one drifts off but so lightly that if a nullifier of wuḍūʾ was to occur he would sense it, then his wuḍūʾ remains intact [regardless of his resting position]. However, if sleep becomes so deep that if a nullifier was to occur he would not sense it, then his wuḍūʾ is invalidated [again, regardless of whether he was standing, sitting, or lying down].

This opinion incorporates all the evidence. The ḥadīth of Ṣafwān ibn ʿAssāl (رضي الله عنه) proves that sleep invalidates the wuḍūʾ, while the ḥadīth of Anas suggests that it does not. We take the sleep of the ṣaḥābah [as in the ḥadīth of Anas] to mean it was light enough (drowsiness etc.) that had a nullifier occurred, they would have sensed it. However, the sleep mentioned in the ḥadīth of Ṣafwān is of the type that had a nullifier occurred, they would not have sensed it (i.e. deep sleep).

This incorporation of all the evidence is also supported by the ḥadīth: “The eyes act to secure the anal sphincter. Should the eyes sleep, the sphincter is loosened.”⁵ Thus, if someone is unable to exert control over his sphincter as a result of deep sleep to the extent that should a nullifier occur, he is not able to sense it, then that sleep is a nullifier (of wuḍūʿ). Otherwise, it is not.

Endnotes:
[1] Authentic: Narrated by Aḥmad 4:239, al-Tirmidhī: 96, and al-Nasāʾī: 158. Graded authentic by al-Tirmidhī, al-Nawawī and Ibn Ḥajar.
[2] Authentic: Narrated by Muslim: 376.
[3] Authentic: Narrated by al-Bazzār in ‘Mukhtaṣir al-Zawāʾid’: 175. Graded authentic by al-Būṣīrī in ‘al-Itḥāf al-Khīrah’ 1:197 and Ibn al-Qaṭṭān in ‘Bayan al-Wahm wa al-Īhām’: 2806.
[4] Nullifier: specifically referring to the ones that can occur in sleep from flatulence, wet dreams, or pre-ejaculate all of which void the wuḍūʾ and may leave the body unintentionally while asleep.
[5] Ḥasan: Narrated by Aḥmad 4:97, al-Ṭabarānī 19:875 and al-Dāraquṭnī 1:160. Graded Ḥasan by al-Mundhirī, Ibn al-Salāḥ, and al-Nawawī. See ‘al-Talkhīṣ’:159 and ‘al-Khulāṣah’:262.

Source: Al-Sharh al-Mumtiʿ 1:275-277
Translated by: Riyāḍ al-Kanadī