Musannaf Ibn Abi Shayba 36:289

Waki’ narrated to us, from Sufyan, from ‘Abd al-Karim, from ‘Ikrimah, who said: When the day of the Trench (al-Khandaq) arrived, a man from the polytheists stood up and said, "Who will duel?" The Messenger of Allah (peace and blessings of Allah be upon him) said, "Stand up, O Zubayr." Safiyyah said, "O Messenger of Allah, my nephew!" He said, "Stand up, O Zubayr." So Zubayr stood up. The Messenger of Allah (peace and blessings of Allah be upon him) said, "Whichever of the two overcomes his companion, he shall kill him." Zubayr overcame him and killed him. Then he came with his spoils, and the Prophet (peace and blessings of Allah be upon him) granted them to him as a gift (nafalahu).