Where does scripture say that it is the only authority? Maybe in John Calvin 12:34
I Corinthians 4:6 And these things, brethren, I have in a figure transferred to myself and to Apollos for your sakes; that ye might learn in us not to think of men above that which is written, that no one of you be puffed up for one against another.
Even Paul commended the Bereans for not trusting even what Paul taught but that they checked with the written word to “see if these things were so”.
Acts 17:11 These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so.