4 Meanwhile, reports of the great events happening among the Khazars reached Rastislav, Prince of Moravia. Aroused by their example, he negotiated with the Emperor Michael III for an evangelizing mission to be sent from Constantinople, and his wish was granted. Thus, the great worth of Cyril and Methodius as seen in their previous accomplishments, together with their zeal for helping their neighbors, caused their selection for the Moravian mission.
Source: Grande Munus (Vatican.va)