Grande Munus §13

¹³ To confirm those things, the pope sent letters to Methodius so that he would not again become subject to the envy of his detractors. For this reason, Methodius performed his assigned duties more vigilantly, confident that he was joined to the pope and to the whole Roman church in a close bond of faith and love. His labor soon produced an exceptional harvest. With the assistance of a priest, he converted prince Boriwoj of Bohemia, then his wife Ludmilla, and before long Christianity spread throughout that land. At the same time he brought the light of the Gospel to Poland; he penetrated Galicia, where he established the episcopal see of Lwdw. Then, as some report, he penetrated into Muscovy and established the episcopal See of Kiev.