Musnad Abu Ya'la al-Mawsili 106:58

Muhammad ibn Bashar narrated to us, Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Ishaq, from Abu 'Ubaydah and another man, from al-Bara', who said: When the Messenger of Allah (peace and blessings of Allah be upon him) wanted to sleep, he would place his right hand under his cheek and say: "O Allah, protect me from Your punishment on the day You gather Your servants."