Surah Al-Fat'h (The Victory), Verse 10

اِنَّ الَّذِیْنَ یُبَایِعُوْنَكَ اِنَّمَا یُبَایِعُوْنَ اللّٰهَ یَدُ اللّٰهِ فَوْقَ اَیْدِیْهِمْ فَمَنْ نَّكَثَ فَاِنَّمَا یَنْكُثُ عَلٰی نَفْسِهٖ وَمَنْ اَوْفٰی بِمَا عٰهَدَ عَلَیْهُ اللّٰهَ فَسَیُؤْتِیْهِ اَجْرًا عَظِیْمًا 

Innal lazeena yubaayi'oonaka innamaa yubaayi'oonal laaha yadul laahi fawqa aydeehim; faman nakasa fainnamaa yankusu 'alaa nafsihee wa man awfaa bimaa 'aahada 'alaihullaaha fasa yu'teehi ajran 'azeemaa

“Verily those who plight their fealty to thee do no less than plight their fealty to Allah: the Hand of Allah is over their hands: then any one who violates his oath, does so to the harm of his own soul, and any one who fulfils what he has covenanted with Allah,- Allah will soon grant him a great Reward.”

Quran 48:10 (Yusuf Ali Translation)