He was born Zaid b. Harith and belonged to the Christian tribe of Bani Kalb from south of Syria. When young, he was carried away and sold into slavery, and was presented by Khadija to Muhammad. Muhammad liked him very much and in the end freed him and adopted Zaid as his son. Muhammad took him to the black stone of the Ka'aba and said: "Bear testimony, all ye that are present. Zaid is my son; I will be his heir and he shall be mine." (W. Muir, The Life of Muhammad, ed. T.H. Wair, p. 35). Hence, Zaid goes by the name Zaid b. Muhammad.
Zaid was one of the first to accept Islam. When Zaid grew up, Muhammad chose Zainab for him, but Zainab rejected the proposal. Zainab was a beautiful Hashemite women of high rank. Later, Muhammad claimed that al-Ahzab 33:36 was revealed to command the obedience of Zainab. Zainab then agreed to marry Zaid.
After a visit of Muhammad to Zainab (where we were told by Muslim traditionists that Muhammad beheld Zainab in her beauty), Zainab began treating Zaid harshly and with disrespect. At the end Zaid divorced her. It is interesting that the marriage planned by Allah and sanctioned in the Qur'an should fail in such a terrible manner. Moreoever, it even set a bad precedent for wives to be disrespectful to their husbands, something that is clearly not allowed in Islam.
Narrated Anas:The hadith is self-serving to Zainab, but interestingly, again, Zainab forgot that her original marriage to Zaid was also sanctioned in the Qur'an.Zaid bin Haritha came to the Prophet complaining about his wife. The Prophet kept on saying (to him), "Be afraid of Allah and keep your wife." Aisha said, "If Allah's Apostle were to conceal anything (of the Quran he would have concealed this Verse." Zainab used to boast before the wives of the Prophet and used to say, "You were given in marriage by your families, while I was married (to the Prophet) by Allah from over seven Heavens." And Thabit recited, "The Verse:-- 'But (O Muhammad) you did hide in your heart that which Allah was about to make manifest, you did fear the people,' (33.37) was revealed in connection with Zainab and Zaid bin Haritha." (Sahih Bukhari 9.516)
Later, Muhammad married Zaid's wife. This caused quite a stir among the Muslim believers. al-Ahzab 33:37 was supposed to be revealed to sanction this marriage. From henceforth, marrying the (divorced) wife of an adopted son is legal. Yet, this verse has no effect, because in Islam, it is forbidden to adopt children (interestingly, in adopting Zaid, Muhammad did what was to be forbidden).
When she learned of Muhammad's marriage to Zainab, Aisha said: "I see your Allah quickly grants you your desire." See also WIVES.
Jesus, however, taught that marrying a divorced woman means committing adultery (Matthew 5:32)