Council of Chalcedon
Council of Chalcedon, Canon XXIV
Definition of faith and received canons; reception qualifications retained; not the full proceedings
Canon XXIV
Monasteries, which have once been consecrated with the consent of the bishop, shall remain monasteries for ever, and the property belonging to them shall be preserved, and they shall never again become secular dwellings. And they who shall permit this to be done shall be liable to ecclesiastical penalties.
Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.
Source: Council of Chalcedon (CCEL)