Zayd ibn Ḥāritha al-Kalbī (زيد بن حارثة الكلبي) (581–629 CE), was an early Muslim, Sahabi and the adopted son of the Islamic prophet, Muhammad. He is commonly regarded as the fourth person to have accepted Islam, after Muhammad's wife Khadija, Muhammad's cousin Ali, and Muhammad's close companion Abu Bakr. Zayd was a slave that Hakim ibn Hizam, Khadija's nephew bought for her at a market in Ukaz. Zayd then became her and Muhammad’s adopted son. This father-son status was later annulled after Muhammad married Zayd’s ex-wife, Zaynab bint Jahsh.
Zayd was a commander in the early Muslim army and led several early military expeditions during the lifetime of Muhammad. Zayd led his final expedition in September 629 CE, and set out to raid the Byzantine city of Bosra. However the Muslim army was intercepted by Byzantine forces and Zayd was subsequently killed at the Battle of Mu'tah.
Zayd is said to have been ten years younger than Muhammad, suggesting a birth-year of 581 He is also said to have been 55 (lunar) years old at his death in 629, indicating a birthdate of 576.
He was born into the Udhra branch of the Kalb tribe in the Najd region, central Arabia. He claimed a pedigree twelfth in descent from Udhra ibn Zayd al-Lat ibn Rufayda ibn Thawr ibn Kalb ibn Wabara. Zayd's mother, Suda bint Thaalaba, was from the Maan branch of the Tayy tribe.
When Zayd was "a young boy of an age at which he could be a servant" he accompanied his mother on a visit to her family. While they were staying with the Maan tribe, horsemen from the Qayn tribe raided their tents and kidnapped Zayd. They took him to the market at Ukkaz and sold him as a slave for 400 dinars .
Zayd's family searched for him, but without success. A lament is attributed to his father, Harithah ibn Sharahil (BaSharahil):
I weep for Zayd, not knowing what became of him.
Is he alive, is he to be expected, or has Death come over him?
By God, I ask yet do not comprehend.
Was it the plain or the mountain that brought about your end?
I wish that I knew: Will you ever return?
In this world only for your coming back I yearn.