(8) باب وَضْعِ الْيَدِ الْيُمْنَى تَحْتَ الْخَدِّ الأَيْمَنِ
Narrated Hudhaifa:
When the Prophet (ﷺ) went to bed at night, he would put his hand under his cheek and then say, "Allahumma bismika amutu wa ahya," and when he got up, he would say, "Al-Hamdu lil-lahi al-ladhi ahyana ba'da ma amatana, wa ilaihi an-nushur."
Reference: Sahih al-Bukhari 6314
In-book reference: Book 80, Hadith 11
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 75, Hadith 326