Orientales Omnes Ecclesias §34

³⁴ But the Roman pontiffs Clement XIV and Pius VII decreed that Catholics of the Ruthenian rite living in Russia could not change over to the Latin rite; and then at length, in the agreement styled Concordia entered into by the Latin and Ruthenian bishops under the auspices of the congregation for the promotion of the faith in 1863, it was laid down that this prohibition bound all Ruthenians.