Canon IX
Of those who on account of their deeds are justly cast forth from the congregation of the Church.
Augustine the bishop, the legate of the Numidian province, said: Deign to enact that if any perchance have been rightly on account of their crimes cast forth from the Church, and shall have been received into communion by some bishop or presbyter, such shall be considered as guilty of an equal crime with them who flee away from the judgment of their own bishop. And all the bishops said: This is the pleasure of all of us.
Let him be excommunicated who communicates with one excommunicated.
This is Canon vii. of the same synod of 387 or 390.