Musnad Ahmad 658:200

Waki’ narrated to us, from Isra’il, from Abu Ishaq, from ‘Abdullah ibn Yazid, from al-Bara’, that the Prophet, peace and blessings of Allah be upon him, when he would go to his bed, he would place his right hand under his cheek and say: "O Allah, protect me from Your punishment on the day You resurrect Your servants," or "gather Your servants."