And where in the Bible does it say that what Paul taught is only in his letters? Indeed his letters were written to communities that had already received the Gospel orally.
If Catholics think that scripture is not the only source they need to provide another source for the apostles teaching the assumption of Mary.
It is called the Tradition of the living church established by Jesus Christ himself. A prime example is the liturgy. We know from Scripture that the institution and celebration of the Eucharist (the Lord's Supper) was essential to the early faith. Yet we find nowhere in the Scriptures how to celebrate it. This is because the early Christians were taught this before the New Testament was even completed. Nor was it thought necessary that such instruction be included in Scripture. It was something that was handed down (traditur). That the teachings of the Church are infallible is shown in Scripture itself.
Then Jesus approached and said to them, “All power in heaven and on earth has been given to me. Go, therefore, and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, until the end of the age.” (Matt. 28:18-20)In the Council of Jerusalem we see the Apostles and presbyters invoking the authority of the Holy Spirit directly, not by appealing to Scripture.And I will ask the Father, and he will give you another Advocate to be with you always, the Spirit of truth, which the world cannot accept, because it neither sees nor knows it. But you know it, because it remains with you, and will be in you. (John 14:16-17)
But if I should be delayed, you should know how to behave in the household of God, which is the church of the living God, the pillar and foundation of truth. (1 Tim 3:15)
Are you intentionally trying to be obtuse? Paul said if they didn't teach it the teacher was to be considered accursed. I'm simply giving you the opportunity to show (since you don't want to rely on scripture alone) where it can be found that the apostles taught the assumption of Mary. Can you not do that? If not I am left with only scripture as a reliable source of what they taught.