Musnad Ahmad 26:136

Waki' narrated to us, from Isra'il, from Abu Ishaq, from Abu 'Ubaydah, from 'Abdullah, who said: When "When the victory of Allah has come and the conquest" (Surah An-Nasr) was revealed to the Messenger of Allah, peace and blessings of Allah be upon him, he would frequently say when he recited it and bowed: "Glory be to You, O Allah, our Lord, and with Your praise. O Allah, forgive me, for You are the Accepting of Repentance, the Merciful," three times.