Surah Al-Fath, Verse 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 عَظِيمٗا١٠

inna alladhīna yubāyiʿūnaka innamā yubāyiʿūna l-laha yadu l-lahi fawqa aydīhim faman nakatha fa-innamā yankuthu ʿalā nafsihi waman awfā bimā ʿāhada ʿalayhu l-laha fasayu'tīhi ajran ʿaẓīman

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.