The Talmud in Masechet Shabbat, asks about the Chanukah blessing on the candles. If all blessings are rabbinic in nature, except for Birkat Hamazon, Grace After Meals, how do we justify the specific wording?
The coinage of that blessing is that we bless G-d Who has sanctified us with His commandments, and commanded us to light the Chanukah candles. If the blessing is rabbinic, how can we say that it was Hashem who commanded us, when it was really, the Rabbis who gave this commandment.
The Sefer Hachinuch lists all of the 613 according to the weekly portion. Commandment number 496 is a negative commandment that states that we must not turn away from the teachings of the Rabbis. In other words, the Torah gave the Rabbis the authority to make enactments and decrees. Therefore, it really is G-d telling us to make the blessing on the Chanukah candles. When we listen to the Rabbis, we are fulfilling a Torah commandment.
This is the source for the power given to the Rabbis. It is their role to safeguard and protect that the Torah not be violated. We must never minimize their authority, and diligently follow that which they teach us.