Sahih Bukhari Volume 7, Book 64, Hadith Number 275.

Narrated By ‘Ali bin Abi Talib : Fatima came to the Prophet asking for a servant. He said, “May I inform you of something better than that? When you go to bed, recite “Subhan Allah’ thirty three times, ‘Alhamdulillah’ thirty three times, and ‘Allahu Akbar’ thirty four times. ‘All added, ‘I have never failed to recite it ever since.” Somebody asked, “Even on the night of the battle of Siffin?” He said, “No, even on the night of the battle of Siffin.”