Musnad Ahmad 26:381

Aswad ibn Amir and Abu Ahmad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from Abu Ubaydah, from Abdullah ibn Mas'ud, who said: When the Prophet, peace and blessings of Allah be upon him, would sleep—Abu Ahmad said: When he would retire to his bed—he would place his right hand under his cheek—Abu Ahmad said: The right one—then he would say: "O Allah, protect me from Your punishment on the day You gather Your servants."