Most probably, Hazrat Abu Zar (radhiyallahu ‘anhu) wanted to hear a detailed description of Rasulullah (sallallahu ‘alaihi wasallam) and the stories regarding him. The report of his brother was too concise and was thus insufficient for him and could not satisfy him.
Hence, he took some provisions and a water bag and personally set out for Makkah Mukarramah. After arriving in Makkah Mukarramah, he met Rasulullah (sallallahu ‘alaihi wasallam) through the intermediary of Hazrat ‘Ali (radhiyallahu ‘anhu). After listening to the message of Rasulullah (sallallahu ‘alaihi wasallam), he immediately embraced Islam, and then proceeded to the Haram where he publicly announced his conversion to Islam.
Hearing his announcement, the kuffaar beat him so severely that he fell to the ground. At that point, Hazrat ‘Abbaas (radhiyallahu ‘anhu) arrived and interceded to save him.
Rasulullah (sallallahu ‘alaihi wasallam) then advised Hazrat Abu Zar (radhiyallahu ‘anhu) to return home and convey the message of Islam to his people. Rasulullah (sallallahu ‘alaihi wasallam) further said, “When you hear that Islam has become dominant in the land, come to us.”
Accordingly, he returned home where he and his brother invited their mother to embrace Islam. Receiving their invitation, she happily and wholeheartedly embraced Islam. He later invited his people, the Ghifaar tribe, towards Islam, and half of them immediately accepted Islam.
(Extracted from Seeratul Mustafa 1/170-171)