Musannaf Ibn Abi Shayba 1:1414

Ishaq ibn Mansur narrated to us, from Mansur ibn al-Aswad, from al-A’mash, from Ibrahim, from Alqama, from Abdullah, who said: The Prophet, peace and blessings of Allah be upon him, would sleep while he was prostrating, and his sleep would not be known except by his breathing (exhaling). Then he would stand up and proceed with his prayer.