The Prophet (ﷺ) performed the fajr prayer, after he said the Salam, he would say:

اَللَّهُمَّ إِنِّي أَسْأَلُكَ يَا اَللَّهُ بِأَنَّكَ الْوَاحِدُ الْأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَّهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوْبِيْ، إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ

Allaahumma inni as'aluka yaa Allahu! Bi'annakal-Waahidul-'Ahadus-Samadul-ladhi lam yalid wa lam yoolad, wa lam yakun lahu kufuwan 'Ahad, 'an taghfirali dhunubi, 'innaka 'Antal-Ghafoorur-Raheem

O Allah, I ask of You, O Allah, as You are the One, the Only, the Self-Sufficient Master, Who begets not nor was He begotten, and there is None equal or comparable to Him, forgive me my sins, for You are the Oft-Forgiving, Most Merciful. Listening to his prayers, The Messenger of Allah (ﷺ) said: "He has been forgiven," three times.

Reference: Sahih. An-Nasa'i: 1301