Tradition
John 21:25
But there are also many other things which Jesus did; which, if they were written every one, the world itself, I think, would not be able to contain the books that should be written.
Context of the passage
John closes his Gospel by acknowledging that his written account, though sufficient for faith, does not exhaust everything Jesus said and did.
Catholic reading
John's own admission that the Gospels do not record everything supports the Catholic belief that authentic apostolic teaching also survived through oral transmission and the Church's living memory, i.e., Sacred Tradition.
Common objection
This is simply hyperbolic language expressing Jesus' greatness, not a statement about extra-biblical doctrine.
Response
Even as hyperbole, the statement concedes that much authentic material about Christ circulated beyond what was written, consistent with Paul's appeal to oral traditions (2 Thess 2:15) rather than an assumption that Scripture alone was ever meant to be exhaustive.