Musannaf Ibn Abi Shayba 18:848

Abu Bakr narrated to us, he said: 'Abdah narrated to us, from Sa'id, from Abu Ma'shar, from Ibrahim, regarding a man who killed a man intentionally, and some of the heirs (of the victim) pardoned him. This was raised to Umar, so he said to Abdullah, "Give your opinion on it." Abdullah said, "You are more entitled to speak on it, O Commander of the Faithful." He (Umar) said, "Abdullah, if some of the heirs pardon him, then there is no retaliation (qisas). It is reduced from him by the portion of the one who pardoned, and they have the remainder of the blood money (diyah)." Umar said, "That is the opinion, and it matches what was in my own soul."