Musar על סנהדרין 219:9

And (Sanhedrin 101a): "R. Chisda said: 'If one argues against his Rabbi, it is as if he would argue against the Shechinah, viz. (Numbers 26:9) ["who strove against Moses and Aaron with the congregation of Korach] when they strove against the L-rd."' And R. Chamma b. Chanina said: "If one quarrels with his Rabbi it is as if he would quarrel with the Shechinah, viz. (Numbers 20:13): "They are the waters of Merivah (contention), whereby the children of Israel strove with the L-rd" [in striving with Moses].'" And, along the same lines: "R. Chaina b. Pappa said: 'If one rails against his Rabbi, it is as if he would rail against the Shechinah, etc.' … R. Avihu said: 'If one arraigns his Rabbi, it is as if he would arraign the Shechinah, etc.'" And the four levels that the Gemara mentions below are meant to apprise us that even thinking alone [against one's Rabbi] is also a great issur.