Musnad Ahmad 28:435

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Sa'd ibn Ibrahim, that he heard a man from Banu Makhzum narrating from his uncle, that Mu'awiyah wanted to take a piece of land belonging to 'Abdullah ibn 'Amr called al-Waht. So he [‘Abdullah] ordered his freedmen, and they put on their equipment and intended to fight. He said: So I came to him and said: "What is this?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'There is no Muslim who is wronged by an injustice, then he fights and is killed, except that he is killed as a martyr.'"