Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: Among the Children of Israel there was retaliation, but there was no diyah among them, so Allah Almighty said to this Ummah: "Retaliation is prescribed for you in the case of those killed: the free for the free, the slave for the slave and the female for the female. Then whoever is pardoned something by his brother" [Surah al-Baqarah: 178]. The meaning of 'afw (pardon) is that they take the diyah and do not kill. "Then let there be a following up in a fair manner and payment to him with excellence. That is an alleviation from your Lord and a mercy" [Surah al-Baqarah: 178], that is, from what was prescribed for those before you. "Whoever transgresses after that will have a painful punishment" [Surah al-Baqarah: 178].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 16034