From Sahih Bukhari (Hadith 2847):
Narrated Jabir bin `Abdullah:When the Prophet (ﷺ) called the people (Sadaqa, a sub-narrator, said, 'Most likely that happened on the day of Al-Khandaq) Az-Zubair responded to the call (i.e. to act as a scout). The Prophet) called the people again and Az-Zubair responded to the call. The Prophet (ﷺ) then said, "Every prophet had a disciple and my disciple is Zubair bin Al-`Awwam
Reference: Bukhari 2847