The Beginning of the Canon?

A lot of people, both Christians and non-Christians, share the misconception that the biblical canon was not developed until hundreds of years after Jesus died. (The more cynical among these think it was a power ploy to stamp out opposing books and doctrines.) Regardless of when the canon was formalized, the idea that certain post-Christ writings were considered sacred can be traced to the New Testament itself:

And regard the patience of our Lord as salvation, just as also our dear brother Paul wrote to you, according to the wisdom given to him, speaking of these things in all his letters. Some things in these letters are hard to understand, things the ignorant and unstable twist to their own destruction, as they also do to the rest of the scriptures. (2 Peter 3:15-16, NET)

This does not answer the question of which writings of Paul (and which other works) are canonical. However, it demonstrates that the apostles (and by extension, most if not all earliest Christians) had a clear conception of "the Scriptures," a compilation of writings that included apostolic texts.

[Update: JLH points me to a similar case in 1 Timothy 5:18.]