Jundub bin 'Abdullah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "Once someone said: 'By Allah! Allah will not forgive such and such (a person).' Thereupon Allah, the Exalted and the Glorious, said: 'Who is he who takes an oath in My Name that I will not grant pardon to so-and-so? I have granted pardon to so-and-so and rendered your good deeds fruitless."'[Muslim].

Jundub bin 'Abdullah (May Allah be pleased with him) reported: The Messenger of Allah (ﷺ) said, "Once someone said: 'By Allah! Allah will not forgive such and such (a person).' Thereupon Allah, the Exalted and the Glorious, said: 'Who is he who takes an oath in My Name that I will not grant pardon to so-and-so? I have granted pardon to so-and-so and rendered your good deeds fruitless."'[Muslim].

وعن جُندب بن عبد الله رضي الله عنه قال‏:‏ قال رسول الله صلى الله عليه وسلم ‏:‏ ‏ "‏قال رجل‏:‏ والله لا يغفر الله لفلان، فقال الله عز وجل‏:‏ من ذا الذي يتألى علي أن لا أغفر لفلان‏!‏ فإني قد غفرت له، وأحبطت عملك‏"‏ ‏(‏‏(‏رواه مسلم‏)‏‏)‏‏.‏