Never Say Allah Will Not Forgive Someone

بِسْــــــــــــــــــمِ اﷲِالرَّحْمَنِ اارَّحِيم

Jundab reported: The Messenger of Allah صلى الله عليه وسلم said: 

A man said, “By Allah! Allah will not forgive so-and-so!” 

So Allah Almighty said, “Who is he who swore by Me that I will not forgive so-and-so? 

I have forgiven him and nullified your good deeds.”

( Sahih Muslim )

عَنْ جُنْدَبٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم 

حَدَّثَ أَنَّ رَجُلاً قَالَ وَاللَّهِ لاَ يَغْفِرُ اللَّهُ لِفُلاَنٍ

 وَإِنَّ اللَّهَ تَعَالَى قَالَ مَنْ ذَا الَّذِي يَتَأَلَّى عَلَىَّ أَنْ لاَ أَغْفِرَ لِفُلاَنٍ

 فَإِنِّي قَدْ غَفَرْتُ لِفُلاَنٍ وَأَحْبَطْتُ عَمَلَكَ

Good Relations and Manners The prohibition of making humanity despair of the Mercy of Allah