Jundab reported: The Messenger of Allah, peace and blessings be upon him, said: “A man said: By Allah, Allah will not forgive this person! Allah the Exalted said: Who is he who swore by me that I will not forgive someone? I have forgiven him and nullified your good deeds.”

Source: Sahih Muslim 2621

Grade: Sahih (authentic) according to Muslim

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

2621 صحيح مسلم كِتَاب الْبِرِّ وَالصِّلَةِ وَالْآدَابِ بَاب النَّهْيِ عَنْ تَقْنِيطِ الْإِنْسَانِ مِنْ رَحْمَةِ اللَّهِ تَعَالَى