Sahih Bukhari Volume 4, Book 56, Hadith 2807

وَقَالَ إِنَّ أُخْتَهُ وَهْىَ تُسَمَّى الرُّبَيِّعَ كَسَرَتْ ثَنِيَّةَ امْرَأَةٍ فَأَمَرَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِالْقِصَاصِ، فَقَالَ أَنَسٌ يَا رَسُولَ اللَّهِ، وَالَّذِي بَعَثَكَ بِالْحَقِّ لاَ تُكْسَرُ ثَنِيَّتُهَا‏.‏ فَرَضُوا بِالأَرْشِ وَتَرَكُوا الْقِصَاصَ، فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّ مِنْ عِبَادِ اللَّهِ مَنْ لَوْ أَقْسَمَ عَلَى اللَّهِ لأَبَرَّهُ ‏"‏‏

His sister Ar-Rubbaya' broke a front tooth of a woman and Allah's Apostle ordered for retaliation. On that Anas (bin An-Nadr) said, "O Allah's Apostle! By Him Who has sent you with the Truth, my sister's tooth shall not be broken." Then the opponents of Anas's sister accepted the compensation and gave up the claim of retaliation. So Allah's Apostle said, "There are some people amongst Allah's slaves whose oaths are fulfilled by Allah when they take them."