That his aunt, Ar-Rubai' broke an incisor tooth of a girl. My aunt's family requested the girl's relatives for forgiveness but they refused; then they proposed a compensation, but they refused. Then they went to Messenger of Allah ﷺ and refused everything except Al-Qisas (i.e. equality in punishment). So Allah's Apostle passed the judgment of Al-Qisas (i.e. equality of punishment). Anas bin Al-Nadr said, "O Messenger of Allah ﷺ! Will the incisor tooth of Ar-Rubai be broken? No, by Him Who sent you with the Truth, her incisor tooth will not be broken." Messenger of Allah ﷺ said, "O Anas! The prescribed law of Allah is equality in punishment (i.e. Al-Qisas.)" Thereupon those people became satisfied and forgave her. Then Messenger of Allah ﷺ said, "Among Allah's Worshippers there are some who, if they took Allah's Oath (for something), Allah fulfill their oaths." (Using translation from Bukhārī 4500)
لَطَمَتِ الرُّبَيِّعُ بِنْتُ النَّضْرِ جَارِيَةً فَكَسَرَتْ ثَنِيَّتَهَا فَطَلَبُوا إِلَيْهِمُ الْعَفْوَ فَأَبَوْا وَعَرَضُوا الْأَرْشَ عَلَيْهِمْ فَأَبَوْا فَأَتَوَا النَّبِيَّ ﷺ فَأَمَرَ بِالْقِصَاصِ فَقَالَ أَنَسُ بْنُ النَّضْرِ يَا رَسُولَ اللهِ أَتُكْسَرُ ثَنِيَّةُ الرُّبَيِّعِ؟ وَالَّذِي بَعَثَكَ بِالْحَقِّ لَا تُكْسَرُ ثَنِيَّتُهَا فَقَالَ النَّبِيُّ ﷺ يَا أَنَسُ كِتَابُ اللهِ الْقِصَاصُ فَرَضِيَ الْقَوْمُ فَعَفَوْا فَقَالَ النَّبِيُّ ﷺ إِنَّ مِنْ عِبَادِ اللهِ مَنْ لَوْ أَقْسَمَ عَلَى اللهِ لَأَبَرَّهُ