Musnad Abu Ya'la al-Mawsili 106:29

Uqbah ibn Mukram narrated to us, Yunus—meaning ibn Muhammad—narrated to us, Yunus ibn Amr narrated to us, from his father, from Abu Ubaydah, from his father Abdullah ibn Mas’ud, that he said: When the Messenger of Allah (peace and blessings of Allah be upon him) would lie down to sleep, he would place his right hand under his right cheek and say: "O Allah, protect me from Your punishment on the day You gather Your servants."