Musnad Abu Ya'la al-Mawsili 152:156

Abu Khaythama narrated to us, Muhammad ibn Khazim narrated to us, from Hajjaj, from Hammad, from Ibrahim, from Alqama, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to sleep lying on his back until he would breathe heavily (snore), then he would stand up and pray, and he would not perform ablution.