It was narrated from 'Abdullah that:
whenever the Prophet ﷺ 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)."

حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، عَنْ إِسْرَائِيلَ، عَنْ أَبِي إِسْحَاقَ، عَنْ أَبِي عُبَيْدَةَ، عَنْ عَبْدِ اللَّهِ، أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا أَوَى إِلَى فِرَاشِهِ وَضَعَ يَدَهُ - يَعْنِي الْيُمْنَى - تَحْتَ خَدِّهِ ثُمَّ قَالَ " اللَّهُمَّ قِنِي عَذَابَكَ يَوْمَ تَبْعَثُ - أَوْ تَجْمَعُ - عِبَادَكَ " .