Sahih Bukhari Volume 4, Book 52, Hadith Number 240.

Narated By Jabir bin ‘Abdullah : On the day of the battle of the Trench, the Prophet wanted somebody from amongst the people to volunteer to be a reconnoitre. Az-Zubair volunteered. He demanded the same again and Az-Zubair volunteered again. Then he repeated the same demand (thrice) and AzZubair volunteered once more. The Prophet then said, ” Every prophet has a disciple and my disciple is Az-Zubair.”