Midrash Tannaim - Deuteronomy 16:19

Another idea, “Do not show favoritism” (Deut. 16:19) - alas this was already said [in Deut 1:17] “You shall not be partial in judgment: hear out low and high alike. Fear no one, for judgment is God's. And any matter that is too difficult for you, you shall bring to me and I will hear it.” Rather, this verse is a warning to the sages when the students sit before them, do not be a teacher to one while setting aside another. If they did this, it is like they worshipped idols. On this it was written after it [in subsequent verses], Justice, justice you shall pursue (Deut 16:20), do not make for yourself an idol. (Deut 16:21). [Translation by Rabbi Rachael Bregman. Edited for gender neutrality]