Musannaf Ibn Abi Shayba 18:1042

Abu Bakr narrated to us, he said: Abd al-Rahim narrated to us, from Isma'il, from 'Amr ibn Dinar, from Tawus, from Ibn Abbas, who said: The Messenger of Allah (peace be upon him) said: "Intentional [killing] is subject to retaliation (qisas), unless the guardian of the slain person pardons."