Musannaf Ibn Abi Shayba 27:418

Muhammad ibn Bishr narrated to us, he said: Mis'ar narrated to us, he said: Abd al-Rahman ibn Milhan narrated to me, he said: Uthman and Umar were mentioned in the presence of Ibn Umar. Ibn Umar said: "Tell me, if you had two camels, one of them strong and the other weak, would you slaughter the weak one?"