Sahih Muslim Book 35, Hadith Number 6580.
Chapter : The reciting of Subbin Allah in the morning and at the time of sleeping.
Abu Huraira reported that Fatima came to Allah’s Apostle (may peace be upon him) and asked for a servant and told him of the hardship of household work. He said: You would not be able to get a servant from us. May I not direct you to what is better than the servant for you? Recite Subhaana Allah thirty-three times, al-Hamdu-li-Allah thirty-three times and Allah-o-Akbar thirty-four times as you go to bed. This hadith has been narrated on the authority of Suhail with the same chain of transmitters.