Sahih Bukhari Volume 4, Book 52, Hadith Number 100.

Narated By Jabir bin ‘Abdullah : When the Prophet called the people (Sadqa, a sub-narrator, said, ‘Most probably that happened on the day of Al-Khandaq) Az-Zubair responded to the call (i.e. to act as a reconnoiter). The Prophet) called the people again and Az-Zubair responded to the call. The Prophet then said, “Every prophet had a disciple and my disciple is Zubair bin Al-‘Awwam.”