22 Pomponius of Dionysiana said: "It is manifest that heretics cannot baptize and give remission of sins, seeing that no power is given to them that they should be able either to loose or bind anything on earth."
23. The answer is: This power is not given to murderers either, that is, to those who hate their brothers. For it was not said to such as these, "whosesoever sins ye remit, they are remitted unto them; and whosesoever sins ye retain, they are retained." And yet they baptize, and both Paul tolerates them in the same communion of baptism, and Cyprian acknowledges them.
Source: On Baptism, Against the Donatists (New Advent)