Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) says: On the day of the Trench, the Messenger of Allah (peace and blessings be upon him) called out to the people, and Sayyiduna al-Zubayr (may Allah be pleased with him) said: Labbayk (Here I am). Then a second time he (peace and blessings be upon him) called the people, and al-Zubayr (may Allah be pleased with him) said: Labbayk. Then a third time he (peace and blessings be upon him) called out, and al-Zubayr (may Allah be pleased with him) said: Labbayk. So he (peace and blessings be upon him) said: "Every prophet has a sincere companion, and my sincere companion is al-Zubayr (may Allah be pleased with him)." (b) Hisham bin Urwah narrates: "My disciple (hawari) is al-Zubayr (may Allah be pleased with him), and he is the son of my paternal aunt."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18441