Sahih Bukhari Volume 005, Book 057, Hadith Number 055.
Narated By 'Ali : Fatima complained of the suffering caused to her by the hand mill. Some Captives were brought to the Prophet, she came to him but did not find him at home 'Aisha was present there to whom she told (of her desire for a servant). When the Prophet came, 'Aisha informed him about Fatima's visit. Ali added "So the Prophet came to us, while we had gone to our bed I wanted to get up but the Prophet said, "Remain at your place". Then he sat down between us till I found the coolness of his feet on my chest. Then he said, "Shall I teach you a thing which is better than what you have asked me? When you go to bed, say, 'Allahu-Akbar' thirty-four times, and 'Subhan Allah thirty-three times, and 'Alhamdu-lillah thirty-three times for that is better for you both than a servant."