On the authority of Jundub (may Allah be pleased with him), who said that the Messenger of Allah (peace and blessings of Allah be upon him) 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 godly man whose previous good deeds were brought to naught through presuming to declare that Allah would not forgive someone's bad deeds.

[Muslim]