Musannaf Ibn Abi Shayba 24:153

Muhammad ibn Kinasa narrated to us, he said: Ishaq ibn Sa'id narrated to us, from his father, who said: Mus'ab ibn al-Zubayr came to Abdullah ibn Umar while he was performing Tawaf between al-Safa and al-Marwa. He said, "Who are you?" He replied, "Your nephew, Mus'ab ibn al-Zubayr." He said, "The master of Iraq?" He said, "Yes. I have come to you to ask you about a people who withdrew their obedience, shed blood, and plundered wealth, so they were fought and defeated. Then they entered [a place] and fortified themselves within it, then they asked for safety and were granted it, then they were killed." He said, "And how many was the number?" He said, "Five thousand." Ibn Umar glorified Allah (said SubhanAllah) at that and said, "O son of al-Zubayr, if a man came to the livestock of al-Zubayr and slaughtered five thousand of them in enmity, do you think he would be a squanderer?" He said, "Yes." He said, "Then you consider it squandering regarding beasts that do not know what Allah is, yet you deem it lawful regarding those who have proclaimed the oneness of Allah (said La ilaha illa Allah) for a single day?"