Sahih Muslim Book 35, Hadith Number 6553.
Chapter : Supplication at the time of sleeping and going to bed.
Abu Huraira reported that Fatima (the daughter of the Holy Prophet) came to Allah’s Apostle (may peace be upon him) and asked for a servant. He said to her: Say: “O Allah, the Lord of the seven heavens”; the rest of the hadith is the same.