Where is it written that we must keep the doctrine taught by our fathers?

Throughout the Bible! In biblical times there were doctors, not everyone could read the Bible, and these doctors and teachers were teaching the Holy Scriptures and the things of God handed down from generation and generation. In the New Testament we find the imprisonment of the apostle Paul who, in prison, He asked to be led, besides the Holy Scriptures, also other writings where you can study. Christian literature is very important for understanding doctrines, we cannot think that we are so good that we can understand everything the Bible says for ourselves. With this attitude, many denominations with different doctrines are born, many divisions, and everyone believes they are right. Without the Fathers we would never have known all the things we know today about early Christianity, we wouldn't even know what the Trinity is, great biblical doctrine, but which we do not find in the Bible itself. In conclusion, nothing is to be rejected, if the ancient teachings conform to Scripture, they are welcome, otherwise we can easily set them aside.