First Council of Nicaea, Canon XI

Concerning those who have fallen without compulsion, without the spoiling of their property, without danger or the like, as happened during the tyranny of Licinius, the Synod declares that, though they have deserved no clemency, they shall be dealt with mercifully.  As many as were communicants, if they heartily repent, shall pass three years among the hearers; for seven years they shall be prostrators; and for two years they shall communicate with the people in prayers, but without oblation.

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