עַל־פִּ֨י הַתּוֹרָ֜ה אֲשֶׁ֣ר יוֹר֗וּךָ וְעַל־הַמִּשְׁפָּ֛ט אֲשֶׁר־יֹאמְר֥וּ לְךָ֖ תַּעֲשֶׂ֑ה לֹ֣א תָס֗וּר מִן־הַדָּבָ֛ר אֲשֶׁר־יַגִּ֥ידֽוּ לְךָ֖ יָמִ֥ין וּשְׂמֹֽאל׃
You shall act in accordance with the instructions given you and the ruling handed down to you; you must not deviate from the verdict that they announce to you either to the right or to the left.
Torah Temimah points to Sanhedrin / Sifrei.
לא תסור. בין לחומרא בין לקולא.
English: You shall not turn aside — whether toward stringency or toward leniency.
The obligation of obedience applies equally to rulings that are strict and rulings that are lenient.
Derash on the verse (Bavli / Sifrei) — not a formal named middah of the 13.
One may not reject a high-court ruling merely because it is lenient or merely because it is stringent.