Musnad Ahmad 1153:473

Abu Salamah narrated to us, Khalid bin Sulayman al-Hadrami narrated to us, from Khalid bin Abi Imran, from Urwah, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) used to, when he sat in a gathering or prayed, speak words. Aisha asked him about the words, and he said: "If he spoke good, it would be a seal upon them until the Day of Resurrection, and if he spoke other than that, it would be an expiation: 'Glory be to You and with Your praise, there is no god but You, I seek forgiveness from Allah, and I repent to Him.'"