Musnad Ahmad 26:198

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Abu Ishaq, from Abu 'Ubaydah, from 'Abdullah, who said: Since the time it was revealed to the Messenger of Allah, may Allah bless him and grant him peace, "When the victory of Allah has come and the conquest," he would frequently say—when he recited it and then bowed in it—"Glory be to You, our Lord, and with Your praise. O Allah, forgive me; indeed, You are the Accepting of repentance, the Merciful," three times.