Musnad Ahmad 658:188

Aswad ibn Amir narrated to us, Israel informed us, from Abu Ishaq, from Abdullah ibn Yazid al-Ansari, from al-Bara’ ibn Azib, that the Prophet, peace and blessings of Allah be upon him, when he would sleep, 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."