Abu Dawud Book 36, Hadith Number 4841.

Narated By AbuBarzah al-Aslami : When the Apostle of Allah (PBUH) intended to get up from the assembly he used to say in the last. Glory be to Thee. O Allah, and I begin with praise of Thee, I testify that there is no god but Thou; I ask Thy pardon, and return to Thee in repentance. The man asked: Apostle of Allah! you utter the words now which you did not do in the past? He replied: (This is an) atonement for what takes place in the assembly.