The Fourth Ecumenical Council.  The Council of Chalcedon 18Canon III.

It has come to [the knowledge of] the holy Synod that certain of those who are enrolled among the clergy have, through lust of gain, become hirers of other men's possessions, and make contracts pertaining to secular affairs, lightly esteeming the service of God, and slip into the houses of secular persons, whose property they undertake through covetousness to manage. Wherefore the great and holy Synod decrees that henceforth no bishop, clergyman, nor monk shall hire possessions, or engage in business, or occupy himself in worldly engagements, unless he shall be called by the law to the guardianship of minors, from which there is no escape; or unless the bishop of the city shall commit to him the care of ecclesiastical business, or of unprovided orphans or widows and of persons who stand especially in need of the Church's help, through the fear of God. And if any one shall hereafter transgress these decrees, he shall be subjected to ecclesiastical penalties.