Sunan Ibn Majah
474
Sahih+3

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)

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi