Sahih Muslim Book 4, Hadith Number 1234.

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

Warrad, the scribe of Mughira b. Shu’ba, reported: Mu’awiya wrote to Mughira: Write to me anything which you heard from the Messenger of Allah (may peace be upon him). So he (Mughira) wrote to him (Mu’awiya): I heard the Messenger of Allah (may peace be upon him) uttering (these words) at the completion of prayer: “There is no god but Allah. He is alone and there is no partner with Him. Sovereignty belongs to Him and to Him is praise due and He is Potent over everything. O Allah ! no one can withhold what Thou givest, or give what Thou withholdest, and riches cannot avail a wealthy person with Thee.”