Musnad Ahmad 789:49

Ya'la narrated to us, Hajjaj ibn Dinar narrated to us, from Abu Hashim, from Rafi' Abu al-'Aliyah, from Abu Barzah al-Aslami, who said: When it was at the end, the Messenger of Allah (peace and blessings of Allah be upon him) would, when he sat in a gathering and wanted to stand 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." They said: "O Messenger of Allah, you are now saying words that you did not use to say in the past." He said: "This is an expiation for what occurs in the gathering."