From Sahih Bukhari (Hadith 7393):
Narrated Abu Huraira:The Prophet (ﷺ) said, "When anyone of you goes to bed, he should dust it off thrice with the edge of his garment, and say: Bismika Rabbi Wada`tu janbi, wa bika arfa'hu. In amsakta nafsi faghfir laha, wa in arsaltaha fahfazha bima tahfaz bihi 'ibadaka-s-salihin
Reference: Bukhari 7393