Musnad Abu Ya'la al-Mawsili 152:302

Abu Khaythama narrated to us, Sa'id ibn Sulayman narrated to us, Mansur ibn Abi al-Aswad narrated to us, he said: al-A'mash narrated to us, from Ibrahim, from 'Alqama, from 'Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to sleep in his prostration, and his sleep would not be known except by his exhaling (breathing sound), then he would stand up for his prayer.