Dua before sleeping — in Your name I lie down
بِاسْمِكَ رَبِّـي وَضَعْـتُ جَنْـبي، وَبِكَ أَرْفَعُـه،...
بِاسْمِكَ رَبِّـي وَضَعْـتُ جَنْـبي، وَبِكَ أَرْفَعُـه، فَإِنْ أَمْسَـكْتَ نَفْسـي فارْحَـمْها، وَإِنْ أَرْسَلْتَـها فاحْفَظْـها بِمـا تَحْفَـظُ بِه عِبـادَكَ الصّـالِحـين
Bismika rabbee wadaAAtu janbee wabika arfaAAuh, fa-in amsakta nafsee farhamha, wa-in arsaltaha fahfathha bima tahfathu bihi AAibadakas-saliheen
Translation
In Your name my Lord, I lie down and in Your name I rise, so if You should take my soul then have mercy upon it, and if You should return my soul then protect it in the manner You do so with Your righteous servants.
When to recite
Recited when lying down to sleep.
Source / Reference
Al-Bukhari 11/126, Muslim 4/2083