Well, since the Apostles knew her personally, and the youngest one took her in as his Mother when Jesus died, I will defer to THEIR teachings on the matter. They may not have written an Epistle or a letter to one of the outlying communities to that effect, but it was an accepted teaching in the first century of Christianity, and is one of those traditions on which our Faith was founded.
That is always a good practice but if you're going to be honest about that you will have to acknowledge that they did not teach that Mary was sinless or was assumed into Heaven.