Tradition
2 Thessalonians 2:15
Therefore, brethren, stand fast; and hold the traditions which you have learned, whether by word, or by our epistle.
Context of the passage
Paul closes his teaching to the Thessalonians on the end times by urging them to hold fast to all he has taught them.
Catholic reading
Paul explicitly places oral teaching ('by word') on equal footing with his written letters, the scriptural basis for the Catholic doctrine that Sacred Tradition and Sacred Scripture together form the single deposit of faith.
Common objection
'Traditions' here just means Paul's own apostolic teaching, later written down in Scripture, not an ongoing extra-biblical Tradition.
Response
Paul distinguishes two channels—oral and written—as both authoritative when he wrote this, showing the apostles considered their spoken teaching, later carried by the Church's living Magisterium, as binding divine revelation, not merely provisional.