Al-Fath 48:10

إِنَّ ٱلَّذِينَ يُبَايِعُونَكَ إِنَّمَا يُبَايِعُونَ ٱللَّهَ يَدُ ٱللَّهِ فَوۡقَ أَيۡدِيهِمۡۚ فَمَن نَّكَثَ فَإِنَّمَا يَنكُثُ عَلَىٰ نَفۡسِهِۦۖ وَمَنۡ أَوۡفَىٰ بِمَا عَٰهَدَ عَلَيۡهُ ٱللَّهَ فَسَيُؤۡتِيهِ أَجۡرًا عَظِيمٗا 10

Indeed, those who pledge allegiance to you, [O Muhammad] - they are actually pledging allegiance to Allah. The hand of Allah is over their hands. So he who breaks his word only breaks it to the detriment of himself. And he who fulfills that which he has promised Allah - He will give him a great reward