Sahih Muslim Book 4, Hadith Number 1242.

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 at the end of every 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.