Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.035B.

Section : Deep sleep that makes a person completely unaware of his surroundings.

If the person did not keep his buttocks firmly seated on the floor while sleeping, he must make a new ablution. Safwan ibn ‘Asal said, “The Prophet, upon whom be peace, used to order us while we were travelling not to take our socks off unless we were in post-sex impurity (i.e. not for defecation, urination or sleep).” (Related by Ahmad, an-Nasa’i and at-Tirmizhi, who graded it sahih.) If one’s buttocks has remained firmly on the floor during his sleep, no new ablution is necessary. This is implied by the hadith of Anas who said, “The companions of the Prophet were waiting for the delayed night prayer until their heads began nodding up and down (from drowsiness and sleep). They would then pray without performing ablution.” (Related by ash-Shaifi, Muslim, Abu Dawud and at-Tirmizhi.) The wording that at-Tirmizhi recorded from the chain of Shu’bah is, “I have seen the companions of the Prophet sleeping to the extent that one could hear some of them snoring. But, they would stand for prayer without a new ablution.” Said Ibn alMubarak, “In our opinion, this happened when they were sitting.”