The Messenger of Allah slept until he was breathing deeply, then he got up and prayed.
Sunan Ibn Majah Book 1, Hadith 209
← Previous Hadith "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).'"

Sunan Ibn Majah Book 1, Hadith 208 Next Hadith → "He would sleep like that while he was sitting up."

Sunan Ibn Majah Book 1, Hadith 210