First Council of Nicaea
First Council of Nicaea, Canon X
Creed and twenty canons; not the full proceedings
Canon X
If any who have lapsed have been ordained through the ignorance, or even with the previous knowledge of the ordainers, this shall not prejudice the canon of the Church; for when they are discovered they shall be deposed.
Translation: Henry R. Percival, The Seven Ecumenical Councils, NPNF II.14 (1900), via CCEL.
Source: First Council of Nicaea (CCEL)