Musannaf Ibn Abi Shayba 21:192

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