Abu Bakr bin Muhammad bin Amr bin Hazm narrates from his father, and he from his grandfather (may Allah be pleased with him), that the Prophet (peace and blessings be upon him) wrote a letter to the people of Yemen; he mentioned the long hadith. In it are also these words: "Whoever deliberately kills a believer, upon him is retaliation (qisas), unless the heirs of the slain person agree (otherwise)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15885