Musannaf Ibn Abi Shayba 33:888

'Affan narrated to us, he said: Ja'far ibn Sulayman narrated to us, he said: Malik ibn Dinar narrated to me, he said: So-and-so narrated to me that 'Amir ibn 'Abd Allah was at al-Rahba, and there was a dhimmi (a non-Muslim subject) being oppressed. He said: So 'Amir threw down his cloak and said: "Do I see the covenant of Allah being violated while I am alive?" Then he rescued him.