|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.