Sifrei Devarim 12

¹(Devarim 1:12) "How can I bear alone your contentiousness" — whence we are taught that they were contentious. If one of them saw his neighbor getting the better of him in judgment, he would say: I have witnesses to bring, I have proofs; tomorrow I will add accusers against you! — thus, "your contentiousness." ²"and your heresy" — whence we are taught that they were "heretics" (against Moses). If Moses left his house (hurriedly to meet them), they would say: Why did the son of Amram leave (so hurriedly)? He is probably not at ease at home. If he was late to leave, they would say: Why isn't the son of Amram leaving? What do you think? He is plotting against you with his wife, as Korach did with his wife! — thus, "and your heresy." ³"and your caviling": They were grumblers. ⁴One would expend a sela (only) to gain two; two, to gain three — thus, "your caviling."