Sunan an-Nasa'i 4756
Anas narrated that:
his paternal aunt broke the front tooth of a girl and the Prophet of Allah decreed retaliation. Her brother, Anas bin An-Nadr, said: "Will you break the front tooth of so and so? No, by the One Who sent you with the truth, the front tooth of so and so will not be broken!" Before that, they had asked her family for forgiveness and blood money. When her brother - who was the paternal uncle of Anas and was martyred at Uhud - swore that oath, the people agreed to forgive. The Prophet said: "There are among the slaves of Allah who, if they swear by Allah, Allah fulfills their oath."
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، وَإِسْمَاعِيلُ بْنُ مَسْعُودٍ، قَالاَ حَدَّثَنَا بِشْرٌ، عَنْ حُمَيْدٍ، قَالَ ذَكَرَ أَنَسٌ أَنَّ عَمَّتَهُ، كَسَرَتْ ثَنِيَّةَ جَارِيَةٍ فَقَضَى نَبِيُّ اللَّهِ صلى الله عليه وسلم بِالْقِصَاصِ فَقَالَ أَخُوهَا أَنَسُ بْنُ النَّضْرِ أَتُكْسَرُ ثَنِيَّةُ فُلاَنَةَ لاَ وَالَّذِي بَعَثَكَ بِالْحَقِّ لاَ تُكْسَرُ ثَنِيَّةُ فُلاَنَةَ ‏.‏ قَالَ وَكَانُوا قَبْلَ ذَلِكَ سَأَلُوا أَهْلَهَا الْعَفْوَ وَالأَرْشَ فَلَمَّا حَلَفَ أَخُوهَا - وَهُوَ عَمُّ أَنَسٍ وَهُوَ الشَّهِيدُ يَوْمَ أُحُدٍ - رَضِيَ الْقَوْمُ بِالْعَفْوِ ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏
"‏ إِنَّ مِنْ عِبَادِ اللَّهِ مَنْ لَوْ أَقْسَمَ عَلَى اللَّهِ لأَبَرَّهُ ‏"‏ ‏.‏