Musnad Ahmad 785:5

Makki ibn Ibrahim narrated to us, Ibn Jurayj narrated to us, he said: Ibrahim ibn Maysarah informed me, from Amr ibn al-Sharid, that he heard him reporting from the Prophet, may Allah bless him and grant him peace, that when he found a man sleeping on his face, with nothing covering his posterior, he would nudge him with his foot and say: "It is the most hateful way of sleeping to Allah, the Almighty and Majestic."