Sahih Muslim Book 4, Hadith Number 1243.

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

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: If anyone extols Allah after every prayer thirty-three times, and praises Allah thirty-three times, and declares His Greatness thirty-three times, ninety-nine times in all, and says to complete a hundred: “There is no god but Allah, having no partner with Him, to Him belongs sovereignty and to Him is praise due, and He is Potent over everything, ” his sins will be forgiven even If these are as abundant as the foam of the sea.