Musnad Ahmad 33:185

Sufyan narrated to us, from Ibn al-Munkadir, that he heard Jabir saying: The Messenger of Allah, peace and blessings of Allah be upon him, called for volunteers among the people on the day of the Trench. Al-Zubayr volunteered. Then he called for volunteers among the people, and al-Zubayr volunteered. Then he called for volunteers among the people, and al-Zubayr volunteered. The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, every prophet has a disciple (hawari), and my disciple is al-Zubayr." Sufyan said: "I heard Ibn al-Munkadir [saying this] in this mosque."