Jundub reported: The Messenger of Allah, peace and blessings be upon him, said:


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

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