Hazrat Hakeem bin Hizaam (Radiyallahu Anhu) purchased Hazrat Zayd (Radiyallahu Anhu) for his aunt, Hazrat Khadijah (Radiyallahu Anha). When Rasulullah (Sallallahu Alaihi Wasallam) married Hazrat Khadijah (Radiyallahu Anha), she gave Hazrat Zayd (Radiyallahu Anhu) to Rasulullah (Sallallahu Alaihi Wasallam) as a gift.

Later on, when Rasulullah (Sallallahu Alaihi Wasallam) was bestowed with Nubuwwah by Allah Ta‘ala, Hazrat Zayd bin Haarithah (Radiyallahu Anhu) brought imaan in him and embraced Islam. (Taareekh-ul-Khamees 2/73)

Hence, Hazrat Zayd bin Haarithah (Radiyallahu Anhu) was among the first Sahaabah who embraced Islam. He embraced Islam in his childhood at the hands of Rasulullah (Sallallahu Alaihi Wasallam). Initially, he was the slave of Rasulullah (Sallallahu Alaihi Wasallam), but after some time, Rasulullah (Sallallahu Alaihi Wasallam) freed him and had taken him as his son. He was thereafter well known by the name “Zaid bin Muhammad” (Zaid, the son of Muhammad).

However, Allah Ta‘ala later on revealed the following verse: Call them by (the names of) their fathers, it is more just in the sight of Allah Ta‘ala. (Surah Ahzaab v. 5)

Through this verse, Allah Ta‘ala prohibited considering the adopted son as one’s real son, and Allah Ta‘ala commanded that the adopted son should be attributed to his biological father. Hence, Rasulullah (Sallallahu Alaihi Wasallam) informed the ummah that Hazrat Zayd (Radiyallahu Anhu) is no longer his son, but his adopted son, and his father is Haarithah. Hence, Hazrat Zayd (Radiyallahu Anhu) should be referred to as “Zaid bin Haarithah”. (Uyoon-ul-Athar 1/112, Saheeh Bukhaari #4782)