In September 629, Muhammad mobilized an army to confront Byzantine forces in Syria, because a Byzantine governor had killed one of his emissaries. He appointed Ja'far as commander of the army and instructed:"If Ja'far is wounded or killed, Zayd ibn Harithah will take over the command.
In September 629, Muhammad assembled an army to face Byzantine forces in Syria due to a governor's assassination of one of his envoys. He appointed Ja'far as the military leader and instructed: "If Ja'far is injured or killed, Zayd ibn Harithah will assume command."