Musannaf Ibn Abi Shayba 37:225

Muhammad ibn Kinasa narrated to us, from Ishaq, from his father, who said: Mus'ab ibn al-Zubayr came to Abdullah ibn Umar while he was performing the circumambulation (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." He said, "I have come to ask you about a people who cast off obedience, shed blood, and gathered wealth, so they were fought and defeated, and they entered a palace and fortified themselves within it. Then they asked for safety and were granted it, and 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, "May Allah prolong your life, O son of al-Zubayr! If a man came to the livestock of al-Zubayr and slaughtered five thousand of them in one morning, would you consider him a squanderer?" He said, "Yes." He said, "Do you consider it squandering regarding beasts that do not know who Allah is, yet you deem it lawful regarding those who have proclaimed the oneness of Allah (said 'La ilaha illa Allah') for even a single day?"