Allah, the Blessed and Exalted, says: "And whoever is killed unjustly, We have certainly given his heir authority (to seek retaliation), so let him not exceed limits in killing (in retaliation)." [Surah al-Isra': 33]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / Q15886