If the Messenger of Allah wanted to sleep he would say:

Bi’ismika ‘allãhumma ‘amootu wa ‘ahyã (43)

and when he awakened from his sleep he used to say:

Al-hamdu lil-lãhi ‘allathee ‘ahyãnã ba`da ma ‘amãtanã, wa ‘ilayhin-nushooru (44)

(43) It is in your name O’ my ‘ilãh 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.

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