Sahih Bukhari Volume 007, Book 064, Hadith Number 274.
Narrated By Ali : Fatima went to the Prophet complaining about the bad effect of the stone hand-mill on her hand. She heard that the Prophet had received a few slave girls. But (when she came there) she did not find him, so she mentioned her problem to 'Aisha. When the Prophet came, 'Aisha informed him about that. 'Ali added, "So the Prophet came to us when we had gone to bed. We wanted to get up (on his arrival) but he said, 'Stay where you are." Then he came and sat between me and her and I felt the coldness of his feet on my abdomen. He said, "Shall I direct you to something better than what you have requested? When you go to bed say 'Subhan Allah' thirty-three times, 'Alhamdulillah' thirty three times, and Allahu Akbar' thirty four times, for that is better for you than a servant."