Abu Dawud Book 2, Hadith Number 1504.
Chapter : What a man should recite after giving the salutations in prayer.
Narated By ‘Ali b. Abi Talib : When the Prophet (PBUH) uttered the salutation at the end of the prayer, he used to say: “O Allah, forgive me my former and later sins, which I have kept secret and what I have done openly, and what I have done extravagance; and what Thou knowest is better than I do. Thou art the Advancer, the Delayer, there is no god but Thou.”