When asked a question, or challenged, (as He often was) Jesus would always appeal to the scriptures (i.e. It is written, What saith the scriptures ..., Search the scriptures, etc.)
ALWAYS? He often times referred to HIMSELF as the Law, such as in Matthew 5 - "You have HEARD (NOTE, He doesn't say, 'you have read ...') it said, but I tell you ..."
I don't recall where Jesus referred to Himself as the Law.
But He did use the formulation you speak of quite often ... "you have heard, ... but I say unto you ..."
He, typically, would use this formulation to correct some point of misinterpretation (or shallow interpretation) ... of God's word to men.
He, typically, would use this formulation to correct some point of misinterpretation (or shallow interpretation) ... of God's word to men.
Misinterpretation? Jesus is setting the bar higher with His RE-interpretation of the Torah. He is going beyond the letter of the Law, which was all that was required previously. Those in the Spirit have a higher standard.
Regards