Council of Chalcedon, Canon XIX

Whereas it has come to our ears that in the provinces the Canonical Synods of Bishops are not held, and that on this account many ecclesiastical matters which need reformation are neglected; therefore, according to the canons of the holy Fathers, the holy Synod decrees that the bishops of every province shall twice in the year assemble together where the bishop of the Metropolis shall approve, and shall then settle whatever matters may have arisen.  And bishops, who do not attend, but remain in their own cities, though they are in good health and free from any unavoidable and necessary business, shall receive a brotherly admonition.

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