Narrated by Hudhaifa (RA): When the Prophet (ﷺ) went to bed at night, he would put his hand under his cheek and then say:

اَللَّهُمَّ بِاسْمِكَ أَمُوْتُ وَأَحْيَا

Allaahumma bismika amutu wa a'h-yaa

O Allah, in Your Name I die and I live.

Reference: Bukhari: 6314