Prophet muhammed PBUH Hadith English

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]