Allah, the Blessed and Exalted, says: "Then whoever (the killer) is pardoned something by his brother, (the claimant) should follow up in a fair manner, and (the killer) should pay him in a good way." [Surah al-Baqarah: 178]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / Q16032