(Devarim 15:5) "Only if you hearken to the voice of the L-rd your G-d": From here they stated: If a man hearkens but a little, he is given to hear much. If he hearkens to the words of Torah, he is given to hear the words of the scribes.
"to observe to do this entire mitzvah": that a "slight one" be as beloved by you as a "weighty one."