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

Lo! those who swear allegiance unto thee (Muhammad), swear allegiance only unto Allah. The Hand of Allah is above their hands. So whosoever breaketh his oath, breaketh it only to his soul's hurt; while whosoever keepeth his covenant with Allah, on him will He bestow immense reward.