Sunan Ibn Majah — The Book of Purification and its Sunnah — Hadith No: 474
Translation
It was narrated that `A'ishah said:"The Messenger of Allah would fall asleep until he was breathing deeply, then he would get up and offer the prayer, and he did not perform ablution." (Hasan) Tanafisi said: "Waki` said: 'She meant while he was prostrating (he would sleep)