عن جندب رضي الله عنه أن رسول الله صلى الله عليه وسلم حدث أن رجلا قال والله لا يغفر الله لفلان وإن الله تعالى قال من ذا الذي يتألى علي أن لا أغفر لفلان فإني قد غفرت لفلان وأحبطت عملك أو كما قال (صحيح مسلم #2621)

Hazrat Jundub (radhiyallahu ‘anhu) has mentioned that Rasulullah (sallallahu ‘alaihi wasallam) narrated the following: A man (from among the previous ummats) said, “By Allah! Allah Ta‘ala will not forgive so-and-so person!” Allah Ta‘ala then said, “Who is this person who takes an oath by My name that I will not forgive so-and-so person? Indeed I have forgiven that person, and I have nullified all your good deeds (i.e. the good deeds of the one who had taken such an oath regarding Allah Ta‘ala not forgiving a certain person).”