70 Peter of Hippo Diarrhytus said: "Since there is one baptism in the Catholic Church, it is clear that a man cannot be baptized outside the Church; and therefore I give my judgment, that those who have been bathed in heresy or in schism ought to be baptized on coming to the Church."
71. There is one baptism in the Catholic Church, in such a sense that, when any have gone out from it, it does not become two in those who go out, but remains one and the same. What, therefore, is recognized in those who return, should also be recognized in those who received it from men who have separated themselves, since they did not lose it when they went apart into heresy.
Source: On Baptism, Against the Donatists (New Advent)