When the surah "When the help of Allah comes and the victory" [Nasr 110:1] was revealed to the Messenger of Allah ﷺ, he would frequently say, whenever he recited it and bowed, "Glory be to You, O Allah, our Lord, and praise be to You. O Allah, forgive me, for You are the Ever-Relenting, the Most Merciful," three times.
حدثنا وكيع عن إسرائيل عن أبي إسحاق عن أبي عبيدة
لما أنزل على رسول الله ﷺ إذا جاء نصر الله والفتح كان يكثر إذا قرأها وركع أن يقول سبحانك اللهم ربنا وبحمدك اللهم اغفر لي إنك أنت التواب الرحيم ثلاثا
Its chain of transmission is daif, because Abu Ubayda did not hear from his father.Ibn Kathir cited it in his tafsir 9:327–328 from this very place, and said, "Ahmad alone narrated it."It is also found in Majma al-Zawaid 3:127, where it is attributed as well to Abu Yaala and Bazzar.