Steve Ray has a list of 35 loaded Questions for "Bible Christians" (quotation marks his)(link to the whole list). This is number 7/35. I'm trying to provide the answers in a common format, for easy reference.
7) Where in the Bible is God’s Word restricted only to what is written down?
Nowhere that we know of.
1) However, the main thing that is called the Word of God is Scripture.
2) And Jesus and the apostles do quote from and rely on the Scriptures as authoritative.
3) And Jesus commands his critics to search the Scriptures to evaluate his ministry.
4) And the Bereans were applauded for searching the Scriptures to evaluate Paul's ministry.