Musnad Ahmad 1155:40

Yazid ibn Harun narrated to us, he said: Hammad ibn Salamah informed us, from ‘Asim ibn Abi al-Najud, from Sawa’ al-Khuza’i, from Hafsah bint ‘Umar, the wife of the Prophet, may Allah bless him and grant him peace, who said: When the Messenger of Allah, may Allah bless him and grant him peace, would retire to his bed, he would place his right hand under his cheek and say: "O Lord, protect me from Your punishment on the day You resurrect Your servants," three times.