Abu Dawud
General Behavior
Book Number: 036 / Hadith Number: 5027
Narated By Hafsah, Ummul Mu'minin : When the Apostle of Allah (pbuh) wanted to go to sleep, he put his right hand under his cheek and would then say three times: O Allah, guard me from Thy punishment on the day when Thou raisest up Thy servants.