On the authority of Jundub (may Allah be pleased with him), who said that the Messenger of Allah (PBUH) 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.