Musannaf Ibn Abi Shayba 17:1236

Ubayd Allah ibn Musa narrated to us, he said: Isra'il informed us, from Abu Ishaq, from Abu Ubaydah, from his father, from the Prophet (peace and blessings of Allah be upon him), that when he would sleep, he would say: "O Allah, protect me from Your punishment on the day You gather Your servants." And he would place his right hand under his cheek.