Qur'an 48:10

إِنَّ ٱلَّذِينَ يُبَايِعُونَكَ إِنَّمَا يُبَايِعُونَ ٱللَّهَ يَدُ ٱللَّهِ فَوْقَ أَيْدِيهِمْ ۚ فَمَن نَّكَثَ فَإِنَّمَا يَنكُثُ عَلَىٰ نَفْسِهِۦ ۖ وَمَنْ أَوْفَىٰ بِمَا عَـٰهَدَ عَلَيْهُ ٱللَّهَ فَسَيُؤْتِيهِ أَجْرًا عَظِيمًا

“Indeed, those who pledge allegiance to you, [O Muḥammad] - they are actually pledging allegiance to Allāh. The hand of Allāh 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 Allāh - He will give him a great reward.”

Al-Fath 48:10 · Saheeh International