Sahih Muslim Book 4, Hadith Number 1241.

Chapter : Desirability of seeking refuge from the torment of the grave.

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.