‘Awn ibn Abdullah reported: Abdullah ibn Mas’ud, may Allah be pleased with him, said, “Allah Almighty will certainly forgive on the Day of Resurrection, with such forgiveness as has never been conceived by the heart of any person.”
Source: Ḥusn al-Ẓan billāh 66
عَنْ عَوْنِ بْنِ عَبْدِ اللَّهِ قَالَ قَالَ عَبْدُ اللَّهِ رضي الله عنه لَيَغْفِرَنَّ اللَّهُ عَزَّ وَجَلَّ يَوْمَ الْقِيَامَةِ مَغْفِرَةً لَمْ تَخْطُرْ عَلَى قَلْبِ بِشْرٍ
66 حسن الظن بالله لابن أبي الدنيا