Jundub reported that Allah's Messenger (ﷺ) stated that a person said:

Allah would not forgive such and such (person). Thereupon Allah the Exalted and Glorious, said: Who is he who adjures about Me that I would not grant pardon to so and so; I have granted pardon to so and so and blotted out his deeds (who took an oath that I would not grant pardon to him).

حَدَّثَنَا سُوَيْدُ بْنُ سَعِيدٍ، عَنْ مُعْتَمِرِ بْنِ سُلَيْمَانَ، عَنْ أَبِيهِ، حَدَّثَنَا أَبُو عِمْرَانَ الْجَوْنِيُّ، عَنْ جُنْدَبٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم حَدَّثَ ‏

‏ أَنَّ رَجُلاً قَالَ وَاللَّهِ لاَ يَغْفِرُ اللَّهُ لِفُلاَنٍ وَإِنَّ اللَّهَ تَعَالَى قَالَ مَنْ ذَا الَّذِي يَتَأَلَّى عَلَىَّ أَنْ لاَ أَغْفِرَ لِفُلاَنٍ فَإِنِّي قَدْ غَفَرْتُ لِفُلاَنٍ وَأَحْبَطْتُ عَمَلَكَ ‏

‏ ‏.‏ أَوْ كَمَا قَالَ ‏.‏