Who was the first governor of Medina after the Hijra?

The first person appointed by the Prophet, peace and blessings be upon him, over Medina after the Hijra was Sa'd ibn 'Ubada. This occurred during the Prophet's first غزوة (military expedition), which was the غزوة of Waddan (also called the غزوة of al-Abwa'). In this غزوة, the Prophet, peace and blessings be upon him, set out in Safar of the first year of the Hijra, intending to confront Banu Damra and the caravan of Quraysh, and he concluded a peace treaty with Banu Damra therein.