Is Scripture the sole rule of faith for Christians? Not according to the Bible. While we must guard against merely human tradition, the Bible contains numerous references to the necessity of clinging to apostolic tradition.
Which we do.
"In times past, God spoke in partial and various ways to our ancestors through the prophets; in these last days, he spoke to us through a son, whom he made heir of all things and through whom he created the universe."
Letter to the Hebrews 1:1-2
There are three stages in the formation of the Gospels: the life and teachings of Jesus, the oral tradition of the Apostles, and the written Gospels.
I have already explained what a valid Tradition is, in a previous post.
Peace