Jesus didn’t tell the apostles to write down everything he had taught them. He simply commanded them to teach it. Much of this teaching later made its way into Sacred Scripture, but every bit of it was and still is considered Sacred Tradition.
John tells us as much at the end of his Gospel: "But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written" (John 21:25). Some of Jesus’ teachings had not yet made it into written form by the date John finished writing his Gospel.
Turning to Luke, we see that the author begins his Gospel by explaining why he is writing it. Luke points out that others have already committed certain things to writing, and he thinks it is a good idea to write down what his reader has already been : (Luke 1:1-4)
Luke, then, commits to writing what has already been taught. That teaching is Sacred Tradition just as surely as Luke’s Gospel will later be recognized as Sacred Scripture.
Moving beyond the Gospels and Acts of the Apostles, we find that Paul provides even more explicit evidence of Sacred Tradition in his writings. Here are three examples: (1 Cor. 11:2), (2 Thess. 3:6), (2 Thess. 2:15).
Now prove from an infallible source that what the Catholic Church teaches as tradition that is not found in scripture is exactly what the apostles called tradition that is not found in scripture.
Also in John, 20:30-31: 30 And many other signs truly did Jesus in the presence of his disciples, which are not written in this book: 31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
So if it has to do with salvation, it is in John. How many teachings must a Catholic believe for eternal life that can not be found in the Gospel of John?