Jundub reported that Messenger of Allah 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 ﷺ 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 .