«كان إذا أراد أن يرقد وضع يده اليمنى تحت خده ثم يقول:» اللهم قني عذابك يوم تبعث عبادك " - ثلاث مرات -
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) intended to sleep, he would place his right hand under his cheek, then say three times: 'O Allah, protect me from Your punishment on the Day You resurrect Your servants.'"
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4656