What did Allah's Apostle (peace be upon him) teach us about equality in punishment as described in the hadith about Ar-Rubai' breaking an incisor tooth?
 Narrated Anas:                     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 Allah's Apostle 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 Allah's Apostle! 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." Allah's Apostle 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 Allah's Apostle said, "Among Allah's Worshippers there are some who, if they took Allah's Oath (for something), Allah fulfill their oaths."