Sahih Muslim
The Book Pertaining to the Merits of the Companions (Allah Be Pleased With Them) of the Holy Prophet (May Peace Be Upon Him) (Kitab Al-Fadail Al-Sahabah)
Book Number: 031 / Hadith Number: 5925
'A'isha 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 nightt? 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 MesseDger. I have come to serve as your sentinel. 'A'isha said: Allah' s Messenger (may peace be upon him) slept (such a sound sleep) that I heard the noise of his snoring.