Sahih Bukhari Volume 001, Book 011, Hadith Number 666.
Narated By Ibn 'Abbas : One night I slept at the house of (my aunt) Maimuna and the Prophet was there on that night. He performed ablution and stood up for the prayer. I joined him and stood on his left side but he drew me to his right and prayed thirteen Rakat and then slept till I heard his breath sounds. And whenever he slept, he used to breathe with audible sounds. The Mu'adhdhin came to the Prophet and he went out and prayed the morning prayer) without repeating the ablution.