Tradition
St. Augustine
On Baptism, Against the Donatists
Book VI › Chapter 20
34. Polianus of Mileum said: "It is right that a heretic should be baptized in the holy Church."
35. Nothing, indeed, could be expressed more shortly. But I think this too is short: It is right that the baptism of Christ should not be depreciated in the Church of Christ.
Source: On Baptism, Against the Donatists (New Advent)