Orientales Omnes Ecclesias §6

⁶ It follows that the action of the metropolitan Isidore of Kiev and the Russians was in harmony with the most ancient tradition of the Ruthenian Church when, in 1439, at the ecumenical council of Florence he signed the decree which solemnly united the Greek to the Latin Church. Nevertheless upon his return from the council, although he was joyfully welcomed at Kiev, his titular see, shortly afterwards at Moscow he was cast into prison and compelled to take to flight and leave the country.