Mishkat al-Masabih
Mishkat al-Masabih 2334
وَعَنْ جُنْدُبٍ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ حَدَّثَ: " أَنَّ رَجُلًا قَالَ: وَاللَّهِ لَا يَغْفِرُ اللَّهُ لِفُلَانٍ وَأَنَّ اللَّهَ تَعَالَى قَالَ: مَنْ ذَا الَّذِي يَتَأَلَّى عَلَيَّ أَنِّي لَا أَغْفِرُ لِفُلَانٍ فَإِنِّي قَدْ غَفَرْتُ لِفُلَانٍ وَأَحْبَطْتُ عَمَلَكَ ". أَوْ كَمَا قَالَ. رَوَاهُ مُسلم
Jundub (may God be pleased with him) reported that the Messenger of God (peace be upon him) related: A man said, ‘By God, God will not forgive so-and-so.’ God Most High said, ‘Who is this who swears on My behalf that I will not forgive so-and-so? I have forgiven him and nullified your deeds’—or words to that effect. Muslim transmitted it. The Arabic source grades it sahih (al-Albani).