Was the Messenger's (PBUH) letter to Al-Mundhir ibn Sawa written in his own hand or by someone else?

The Prophet, peace and blessings be upon him, was illiterate; he neither wrote nor read. He employed scribes to write down the revelation, letters, and treaties for him. He sent letters to kings, among whom was Al-Mundhir ibn Sawa, and sealed them with his ring, which was engraved with "Muhammad is the Messenger of Allah."