Hadith: Who is that who swears that I will not forgive so-and-so? Indeed, I have forgiven him and rendered worthless your deeds

عن جندب بن عبد الله رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "قال رجل: والله لا يغفر الله لفلان، فقال الله: من ذا الذي يتألى عليَّ أن لا أغفر لفلان؟ إني قد غفرت له، وأحبطت عملك". وفي حديث أبي هريرة: أن القائل رجل عابد، قال أبو هريرة: "تكلم بكلمة أوبقت دنياه وآخرته".
[صحيح] - [رواه مسلم]

Jundub ibn ‘Abdullāh (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "A man said: 'By Allah, Allah will not forgive so-and-so.' Thereupon, Allah said: 'Who is that who swears that I will not forgive so-and-so? Indeed, I have forgiven him and rendered worthless your deeds.'" In a similar Hadīth reported by Abu Hurayrah: ''The sayer was a worshiper.'' Abu Hurayrah said: ''He said a word that ruined his life and Hereafter.’'
Sahih/Authentic. - [Muslim]

Explanation

Warning against the dangers of the tongue, the Prophet (may Allah's peace and blessings be upon him) says that a man once swore that Allah would not forgive a sinner. It was as if he was passing a judgment that limits Allah's authority, believing in his credit and the sinner's disgrace. His conceit implies his rudeness toward Allah, a practice that subjected him to a miserable fate in life and the Hereafter.