Musannaf Ibn Abi Shayba 27:570

Abu Usama narrated to us, from Ibn 'Awn, from Umayr ibn Ishaq, that Hamza used to fight in front of the Prophet, may Allah bless him and grant him peace, with two swords, and he would say: "I am the lion of Allah and the lion of the Messenger of Allah, may Allah bless him and grant him peace."