1 Cyprian to Jubaianus his brother, greeting. You have written to me, dearest brother, wishing that the impression of my mind should be signified to you, as to what I think concerning the baptism of heretics; who, placed without, and established outside the Church, arrogate to themselves a matter neither within their right nor their power. This baptism we cannot consider as valid or legitimate, since it is manifestly unlawful among them; and since we have already expressed in our letters what we thought on this matter, I have, as a compendious method, sent you a copy of the same letters, what we decided in council when very many of us were present, and what, moreover, I subsequently wrote back to Quintus, our colleague, when he asked about the same thing.
And now also, when we had met together, bishops as well of the province of Africa as of Numidia, to the number of seventy-one, we established this same matter once more by our judgment, deciding that there is one baptism which is appointed in the Catholic Church; and that by this those are not re-baptized, but baptized by us, who at any time come from the adulterous and unhallowed water to be washed and sanctified by the truth of the saving water.
Source: The Epistles of Cyprian (New Advent)