Second Council of Nicaea, Canon IX

That none of the books containing the heresy of the traducers of the Christians are to be hid.

All the childish devices and mad ravings which have been falsely written against the venerable images, must be delivered up to the Episcopium of Constantinople, that they may be locked away with other heretical books.  And if anyone is found hiding such books, if he be a bishop or presbyter or deacon, let him be deposed; but if he be a monk or layman, let him be anathema.

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