late in the six year a.h., on his return from hudaibiyah, the prophet [pbuh] decided to send messages to the kings beyond arabia calling them to islam. in order to authenticate the credentials of his envoys, a silver seal was made in which were graven the words: "muhammad the messenger of allâh" in the following formation: [sahih al-bukhari 2/872,873]

envoys were chosen on the basis of their experience and knowledge, and sent on their errands in muharram in the year 7 a.h., a few days before heading for khaibar. [rahmat-al-lil'alameen 1/171]