Musannaf Ibn Abi Shayba 36:216

Abu Usama narrated to us, from Ibn 'Awn, from Umayr ibn Ishaq, who said: Hamza used to fight in front of the Messenger of Allah, may Allah bless him and grant him peace, on the day of Uhud with two swords, and he would say, "I am the Lion of Allah." He said: He began advancing and retreating, then he stumbled and fell onto his back, lying flat. The armor pulled up and his stomach was exposed. The Abyssinian slave saw him, so he threw a spear or a javelin at him, and he ripped him open with it.