Mishnayos Sanhedrin Perek 2 Mishnah 5

One may not ride on the king’s horse, and one may not sit on his throne, and one may not use his scepter, and one may not see him when he is having his hair cut, nor when he is naked, nor when he is in the bathhouse, as it is stated: “You shall set a king over you” (Deuteronomy 17:15), meaning, ensure that his fear should be upon you. All of these actions would lessen one’s fear of and reverence for the king.

אֵין רוֹכְבִין עַל סוּסוֹ, וְאֵין יוֹשְׁבִין עַל כִּסְאוֹ, וְאֵין מִשְׁתַּמְּשִׁין בְּשַׁרְבִיטוֹ, וְאֵין רוֹאִין אוֹתוֹ כְּשֶׁהוּא מִסְתַּפֵּר וְלֹא כְשֶׁהוּא עָרֹם וְלֹא בְבֵית הַמֶּרְחָץ, שֶׁנֶּאֱמַר (שם) שׂוֹם תָּשִׂים עָלֶיךָ מֶלֶךְ, שֶׁתְּהֵא אֵימָתוֹ עָלֶיךָ: