Hadith: When the Messenger of Allah wanted to sleep, he would put his right hand under his cheek, and then say: O Allah, safeguard me from Your torment on the day You resurrect Your slaves

عن حذيفة بن اليمان رضي الله عنهما وحفصة بنت عمر بن الخطاب رضي الله عنهما أن رسول الله صلى الله عليه وسلم كَانَ إِذَا أَرَادَ أَنْ يَرْقُدَ، وَضَعَ يَدَهُ اليُمْنَى تَحْتَ خَدِّهِ، ثُمَّ يَقُولُ: «اللَّهُمَّ قِني عذابك يوم تبعث عبادك». وفي رواية: أنه كان يقوله ثلاث مرات.
[صحيح دون قوله: "ثلاث مرات"] - [حديث حذيفة: رواه التُرمذي وأحمد. حديث حفصة: رواه أبو داود والنسائي في الكبرى وأحمد. والزيادة في حديث حفصة]

Hudhayfah ibn al-Yamān (may Allah be pleased with him) and Hafsah bint ‘Umar ibn al-Khattāb (may Allah be pleased with him) reported that when the Messenger of Allah wanted to sleep, he would put his right hand under his cheek, and then say: ''O Allah, safeguard me from Your torment on the day You resurrect Your slaves.''
Sahih/Authentic. - [At-Tirmidhi]

Explanation

This Hadīth addresses the Prophet's practical and verbal Sunnah concerning sleeping. As for the practical Sunnah here, the Prophet would sleep on his right side, since he would put his right hand under his cheek, as Hudhayfah ibn al-Yamān described. So, one might want to emulate the Prophet by putting his right hand under his cheek, or only sleep on the right side, as mentioned in other narrations. The agreed upon practice, however, is to sleep on the right side. It would take the Prophet a while as understood from “and then” to make the supplication, which is the verbal Sunnah upon sleeping. It is fine that one talks with one's spouse, and then say the prayer. Here the Prophet would invoke Allah to protect him from torment on the Day of Resurrection. In this respect, seeking Allah's protection can either be seeking His benevolence and favor, or Allah's assistance to enable him do what keeps him away from torment and admit him to Paradise, or both things together. Indeed, he meant the two things, because the rule is that general indications of a text prevail unless they are specified by other texts. The word 'torment' connotes all types of punishment expected on Doomsday. It suggests initial admission to Hell on a day which is so terrible and scary. Torment is also related to Allah to indicate its horrific magnitude. This relation further implies that one should entrust things to the Lord, Who manages the conditions of His slaves, being the dominating sovereign. Finally, there is subtle connection between sleep (minor death) and resurrection (as a post-death event), which calls for thinking about the logic and beauty of the Prophet's words.