Four words going to sleep, and what he said waking
Hudhayfah gives both halves.
اللَّهُمَّ بِاسْمِكَ أَمُوتُ وَأَحْيَا
Allāhumma bismika amūtu wa aḥyā
“O Allah, in Your name I die and I live.”
Where this comes from
Hand under the cheek, and this. And when he woke: praise be to Allah who gave us life after He caused us to die, and to Him is the rising. Sleep is called dying and waking is called being brought back, twice, in the same night.
The full narration
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
Source
Ṣaḥīḥ al-Bukhārī · 6314
Ṣaḥīḥ
- Agreed upon as authentic (al-Bukhārī / Muslim)