Sahih Muslim 2621
Sahih Muslim · Book 45: The Book of Virtue, Enjoining Good Manners, and Joining of the Ties of Kinship · The Prohibition Of Making Others Despair Of The Mercy Of Allah
Jundub reported that Allah's Messenger (ﷺ) stated that a person said:
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 .
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).