Musnad Ahmad 658:224

Ishaq ibn Yusuf narrated to us, Sufyan narrated to us, from Abu Ishaq, from al-Bara’, that the Messenger of Allah (peace and blessings of Allah be upon him) used to place his right hand under his cheek when he went to sleep, and he would say: "O Allah, protect me from Your punishment on the day You resurrect Your servants."