Musannaf Ibn Abi Shayba 28:743

Ghundar narrated to us, from Shu'bah, from Khalid ibn Ja'far, from al-Hasan, that a captive was brought to al-Hajjaj. He said to 'Abdullah ibn 'Umar, "Stand up and kill him." Ibn 'Umar said, "We were not commanded with this. Allah says: 'Until, when you have slaughtered them, then secure their bonds; then either favor afterwards or ransom.'"