Musnad Ahmad 789:7

Abdullah ibn Numayr narrated to us, Hajjaj informed us, from Abu Hashim al-Wasiti, from Abu Barzah al-Aslami, who said: The Messenger of Allah (peace and blessings of Allah be upon him) would, at the end of a gathering, when it had been long and he stood up, say: "Glory be to You, O Allah, and with Your praise. I bear witness that there is no god but You. I seek Your forgiveness and I repent to You." Some of us said to him: "Indeed, this is a statement we have not heard from you in the past." The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is an expiation for what occurs in the gathering."