Sunan Ibn Majah
3877
Sahih+2

It was narrated from 'Abdullah that:whenever the Prophet (saas) went to his bed, he would put his hand - meaning his right hand - beneath his cheek then say: "Allahumma qini 'adhabaka yawm tab'athu - [or: tajma'u] - 'ibadaka (O Allah, save me from Your punishment on the Day when You resurrect - or gather - Your slaves)

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi