2 Peter 3:14-16 Consider also that our Lord’s patience brings salvation, just as our beloved brother Paul also wrote you with the wisdom God gave him. He writes this way in all his letters, speaking in them about such matters. Some parts of his letters are hard to understand, which ignorant and unstable people distort, as they do the rest of the Scriptures, to their own destruction.…
So Peter endorsed Paul and his letters as Scripture.
Further, Jesus emphasized the importance of relying on Scripture ("Moses") to know anything else:
John 5:46-47 If you believed Moses, you would believe me, for he wrote about me. But since you do not believe what he wrote, how are you going to believe what I say?"
And from Luke 16:31, Jesus said:
"“He said to him, ‘If they do not listen to Moses and the Prophets, they will not be convinced even if someone rises from the dead.’"
"Moses and the Prophets" was a common term for the Scriptures in that day.