The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1241
Ka'b b. 'Ujra reported Allah's Messenger (may peace be upon him) as saying: There are certain ejaculations, the repeaters of which or the performers of which after every prescribed prayer will never be caused disappointment:" Glory be to Allah" thirty-three times." Praise be to Allah" thirty-three times, and" Allah is most Great" thirty-four times.