First Council of Nicaea, Canon III

The great Synod has stringently forbidden any bishop, presbyter, deacon, or any one of the clergy whatever, to have a subintroducta dwelling with him, except only a mother, or sister, or aunt, or such persons only as are beyond all suspicion.

Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.