Musnad Abu Ya'la al-Mawsili 210:27

Zuhayr narrated to us, Yazid bin Harun narrated to us, Hammad bin Salamah informed us, from ‘Asim bin Abi al-Najud, from Sawa’ al-Khuza’i, from Hafsah bint ‘Umar, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would go to his bed, he would place his right hand under his cheek and say: "My Lord, protect me from Your punishment on the day You resurrect Your servants," three times.