"[T]he custom [of not rebaptizing converts] . . . may be supposed to have had its origin in Apostolic Tradition, just as there are many things which are observed by the whole Church, and therefore are fairly held to have been enjoined by the Apostles, which yet are not mentioned in their writings" (On Baptism, Against the Donatists 5:23[31] [A.D. 400]).
"But the admonition that he [Cyprian] gives us, 'that we should go back to the fountain, that is, to Apostolic Tradition, and thence turn the channel of truth to our times,' is most excellent, and should be followed without hesitation" (ibid., 5:26[37]).
"But in regard to those observances which we carefully attend and which the whole world keeps, and which derive not from Scripture but from Tradition, we are given to understand that they are recommended and ordained to be kept, either by the Apostles themselves or by plenary [ecumenical] councils, the authority of which is quite vital in the Church" (Letter to Januarius [A.D. 400]).