Then beginning with Moses and with all the prophets, He explained to them the things concerning Himself in all the Scriptures. Luke 24:27
44Now He said to them, “These are My words which I spoke to you while I was still with you, that all things which are written about Me in the Law of Moses and the Prophets and the Psalms must be fulfilled.†45Then He opened their minds to understand the Scriptures, Luke 24:44-45
You, however, continue in the things you have learned and become convinced of, knowing from whom you have learned them, 15and that from childhood you have known the sacred writings which are able to give you the wisdom that leads to salvation through faith which is in Christ Jesus. 16All Scripture is inspired by God and profitable for teaching, for reproof, for correction, for training in righteousness; 17so that the man of God may be adequate, equipped for every good work. 2 Timothy 3:14-17
When Jesus was tempted by Satan, Christ appealed to the Scriptures.
When Peter delivered his sermon in Acts he appealed to the Scriptures.
When Paul refuted the false teachings in Galatians he appealed to the Scriptures.
God gave His commandments to Moses in written form.
The writer of Hebrews appealed to the Scriptures.
More could be listed but the premise is clear....Scripture is to be the final source we appeal to for authority.
As John put it:
30Therefore many other signs Jesus also performed in the presence of the disciples, which are not written in this book; 31but these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name. John 20:30-31 NASB
13These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. 1 John 5:13 NASB
Paul's writings were considered Scripture as well.