On the authority of Jundub (may Allah be pleased with him), who said that the Messenger of Allah ﷺ related:
A man said: By Allah, Allah will not forgive So-and-so. At this Allah the Almighty said: Who is he who swears by Me that I will not forgive So-and-so? Verily I have forgiven So-and-so and have nullified your [own good] deeds (1) (or as he said [it]).
(1) A similar Hadith, which is given by Abu Dawud, indicates that the person referred to was a goldly man whose previous good deeds were brought to nought through presuming to declare that Allah would not forgive someone's bad deeds.
It was related by Muslim.
عَنْ جُنْدُبٍ رَضِيَ اللهُ عَنْهُ : أَنَّ رَسُولَ اللهِ صَلَّى الله عَلَيْهِ وَ سَلَّمَ حَدَّثَ
(أَنْ رجُلاً قال : واللهِ لا يَغْفِرُ اللهُ لِفُلانٍ وإِنَّ اللهَ تَعَالَى قَالَ : مَنْ ذَا الَّذِي يَتَأَلَّى عَلَيَّ أَنْ لا أَغْفِرَ لِفُلان،فَإِنِّي قَدْ غَفَرْتُ لِفُلانٍ، وأَحْبَطْتُ عَمَلَكَ (أَوْ كَمَا قَال