Musannaf Ibn Abi Shayba 28:874

Marwan ibn Mu'awiyah narrated to us, from Humayd, from Anas, who said: We besieged Tustar, and al-Hurmuzan surrendered upon the judgment of Umar. Abu Musa sent him with me. When we arrived before Umar, al-Hurmuzan remained silent and did not speak. Umar said: "The speech of the living or the speech of the dead?" He said: "Then speak, for there is no harm." He said: "We and you, O assembly of the Arabs, as long as God left [the matter] between us and you, we used to kill you and drive you away. But when God was with you, we had no power against you." He said: So Umar said: "What do you say, O Anas?" I said: "O Commander of the Faithful, I left behind a severe force and a great number. If I kill him, the people will despair of life and it will be more intense for their force, and if I spare him, the people will have hope." He said: "O Anas, spare the killer of al-Bara' ibn Malik and Mujza'ah ibn Thawr!" When I feared that he would execute him, I said to him: "You have no way to kill him." Umar said: "Why? Did I not give you [permission] to take from him?" I said: "You did not, but you said to him: 'Speak, for there is no harm.'" He said: "You shall bring someone who testifies with you, or I will begin with your punishment." He said: So I went out from his presence, and there was al-Zubayr ibn al-Awwam, who had memorized what I had memorized, so he testified before him, and he spared him. Al-Hurmuzan accepted Islam, and he [Umar] assigned him a stipend.