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)." (Using translation from Ibn Mājah 3877)

أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا نَامَ وَضَعَ يَدَهُ الْيُمْنَى تَحْتَ خَدِّهِ وَقَالَ اللهُمَّ قِنِي عَذَابَكَ يَوْمَ تَبْعَثُ عِبَادَكَ