Leviticus Rabbah 24:5

R. Hiyya taught, "[The statement, ‘Say to all the congregation of the people of Israel’ (Lev. 19:2)] teaches that the entire passage was stated on the occasion of the gathering [of the entire assembly.] "And what is the reason that it was stated on the occasion of the gathering [of the entire assembly]? Because the majority of the principles of the Torah depend upon [what is stated in this chapter of the Torah.]" R. Levi said, "It is because the Ten Commandments are encompassed within its [teachings].  ¼ "‘You shall not covet’ [Ex. 20:14], and here it is written, ‘And you shall love your neighbor as yourself’" (Lev. 19:18).