Hadeeth Qudsi 31:

On the authority of Jundub (mayAllah be pleased with him), who said that the Messenger of Allah related:

A man said: By Allah, Allah willnot forgive So-and-so. At this Allah the Almighty said: Who is he whoswears by Me that I will not forgive So-and-so? Verily I have forgivenSo-and-so and have nullified your [own good] deeds (1) (or as he said[it]).

(1) A similar Hadeeth, which isgiven by Abu Dawud, indicates that the person referred to was a goldlyman whose previous good deeds were brought to nought through presumingto declare that Allah would not forgive someone’s bad deeds.

It was related by Muslim.