It is brought in the Talmud (Sanhedrin 91b):

Whoever withholds a law from the mouth of a student, it is as if he robs him of his ancestral heritage, for it is stated (Deuteronomy 33:4): “The Torah that Moshe charged us with is the heritage [of the congregation of Yaakov].”

Thus when Moshe saw the Reubenites and Gadites disheartening the Children of Israel from their ancestral heritage, on account of this he called them “a band of sinful men” (Numbers 32:14). Onkelos renders this into Aramaic as: “students of guilty men.”

“This is something you acquired from your teachers. Your teachers stole laws from you, and laws are the ancestral heritage,” as explained above. “And now, you are following in the ways of your ancestors—you too seek to rob Israel of its ancestral heritage.”

And in the first chapter of Bava Batra (8b): “those who turn the many to righteousness are like the stars” (Daniel 12:3)—this refers to the teachers of schoolchildren.

Thus now, when [Moshe] saw them causing others to be guilty, disheartening the Children of Israel, [he concluded] that their teachers, too, most likely did not “turn the many to righteousness,” but instead caused the many to be guilty. It was because of this that he called them “a band of sinful men,” which Onkelos renders into Aramaic as: “students of guilty men.”