Narrated Mihjan ibn al-Adra':

The Messenger of Allah ﷺ entered the mosque and saw a man who had finished his prayer, and was reciting the tashahhud saying: O Allah, I ask you, O Allah, the One, the eternally besought of all, He begetteth not, nor was He begotten, and there is none comparable unto Him, that you may forgive me my sins, you are Most Forgiving, Merciful.

He (the Prophet) said: He was forgiven (repeating three times.)

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ عَمْرٍو أَبُو مَعْمَرٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، حَدَّثَنَا الْحُسَيْنُ الْمُعَلِّمُ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ، عَنْ حَنْظَلَةَ بْنِ عَلِيٍّ، أَنَّ مِحْجَنَ بْنَ الأَدْرَعِ، حَدَّثَهُ قَالَ دَخَلَ رَسُولُ اللَّهِ ﷺ الْمَسْجِدَ فَإِذَا هُوَ بِرَجُلٍ قَدْ قَضَى صَلاَتَهُ وَهُوَ يَتَشَهَّدُ وَهُوَ يَقُولُ اللَّهُمَّ إِنِّي أَسْأَلُكَ يَا اللَّهُ الأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوبِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ . قَالَ فَقَالَ
" قَدْ غُفِرَ لَهُ قَدْ غُفِرَ لَهُ " . ثَلاَثًا .