Hazrat ‘Urwah bin Zubair (rahimahullah) narrates the following:
On one occasion, Shaitaan caused a false rumor to spread that Rasulullah (sallallahu ‘alaihi wasallam) had been seized and captured by the Kuffaar in the upper area of Makkah Mukarramah. When he heard this rumor, Hazrat Zubair (radhiyallahu ‘anhu), who was only twelve years old at the time, immediately set out, parting through the people with his sword in his search for Rasulullah (sallallahu ‘alaihi wasallam). All who saw Hazrat Zubair (radhiyallahu ‘anhu) were surprised and remarked, “The youngster is carrying a sword!” When he reached Rasulullah (sallallahu ‘alaihi wasallam), Rasulullah (sallallahu ‘alaihi wasallam) asked him, “What is the matter, O Zubair?” Hazrat Zubair (radhiyallahu ‘anhu) replied by informing Rasulullah (sallallahu ‘alaihi wasallam) of the false rumor that had spread and said, “I came to strike those who seized you with my sword.”
In another narration, reported by the son of Hazrat ‘Urwah (rahimahullah), Hazrat Zubair (radhiyallahu ‘anhu) was the first person to draw out his sword for the sake of Allah Ta‘ala. This narration further mentions that when Rasulullah (sallallahu ‘alaihi wasallam) heard this reply of Hazrat Zubair (radhiyallahu ‘anhu), he made du‘aa for him.
Note: There are different narrations regarding the age of Hazrat Zubair (radhiyallahu ‘anhu) at the time of his accepting Islam. There are two different views that have been narrated from ‘Urwah (rahimahullah). Abul Aswad narrates from ‘Urwah (rahimahullah) that Hazrat Zubair (radhiyallahu ‘anhu) was twelve years old when he accepted Islam, and Hishaam bin ‘Urwah (rahimahullah) narrates from his father, ‘Urwah (rahimahullah), that he was sixteen years old. As for Hishaam bin ‘Urwah (rahimahullah), his view is that Hazrat Zubair (radhiyallahu ‘anhu) was fifteen years old when he accepted Islam. There is also a view of Hazrat Zubair (radhiyallahu ‘anhu) being eight years old at the time of accepting Islam. He accepted Islam shortly after Hazrat Abu Bakr (radhiyallahu ‘anhu).
(Siyar A’laam min Nubalaa 3/27, Usdul Ghaabah 2/210, Musannaf ‘Abdur Razzaaq #9647, Musannaf ibn Abi Shaibah #19869)