Authentic Supplication Number 024.

If the Messenger of Allah  wanted to sleep he would say Bi'ismika 'allahumma 'amootu wa 'ahya (43) and when he awakened from his sleep he used to say: Al-hamdu lil-lahi 'allathee 'ahyana ba`da ma 'amatana, wa 'ilayhin-nushooru (44).

Footnotes.
(43) It is in your name O' my 'ilah that I die and I live.
(44) All praise be to Allah Who has caused us life after He had caused us death, and to Him is the return.

Reporters.
Agreed upon by Bukhari and Muslim, and Al-Albani included it in The Authentic of Good Sayings as #24.