Allah's Apostle (ﷺ) entered the mosque. Then a person is reciting Tashahhud towards the end of the prayer. He said -
اَللَّهُمَّ إِنِّي أَسْأَلُكَ يَا اللَّهُ بِأَنَّكَ الْوَاحِدُ الْأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوبِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ
اَللّٰهُمَّ اِنِّيْ اَسْاَلُكَ يَا اَللّٰهُ بِاَنَّكَ الْوَاحِدُ الْاَحَدُ الصَّمَدُ الَّذِيْ لَمْ يَلِدْ وَلَمْ يُوْلَدْ وَلَمْ يَكُنْ لَّهٗ كُفُوًا اَحَدٌ اَنْ تَغْفِرَ لِيْ ذُنُوْبِيْ اِنَّكَ اَنْتَ الْغَفُوْرُ الرَّحِيْمُ
Allaahumma inni as'aluka ya Allahu bi annakal Wahidul-Ahad us-Samadulla-dhi lam yalid wa lam yowlad, wa lam yakun lahu kufuwan ahad, an taghfirali dhunubi, innaka antal-Ghafurur-Rahim
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.
The Messenger of Allah (ﷺ) said: "He has been forgiven," three times.