Musnad Ahmad 1155:43

'Abd al-Samad narrated to us, Aban—meaning Ibn Yazid al-'Attar—narrated to us, he said: 'Asim narrated to us, from Ma'bad ibn Khalid, from Sawa' al-Khuza'i, from Hafsa bint 'Umar: "That the Messenger of Allah (peace and blessings of Allah be upon him), when he wanted to sleep, would place his right hand under his right cheek, then he would say: 'O Allah, protect me from Your punishment on the day You resurrect Your servants,' three times. And his right hand was for his food and his drink, and his left hand was for the rest of his needs."