Sahih Muslim Book 31, Hadith Number 5925.
Chapter : Merits of Sa’d b. Abi Waqqis (Allah be pleased with him).
‘Aisha reported that Allah’s Messenger (may peace he upon him) lay on bed during one night and said: Were there a pious person from amongst my companions who should keep a watch for me during the night? She said: We heard the noise of arms, whereupon Allah’s Messenger (may peace be upon him) said: Who is it? And Sa’d b. Abi Waqqas said: Allah’s Messenger. I have come to serve as your sentinel. ‘Aisha said: Allah’ s Messenger (may peace be upon him) slept (such a sound sleep) that I heard the noise of his snoring.