Sanhedrin 19B

נפנה לימינו ⁠– כבשו פניהם בקרקע. נפנה לשמאלו ⁠– וכבשו פניהם בקרקע. אמר להן שמעון בן שטח: בעלי מחשבות אתם, יבא בעל מחשבות ויפרע מכם! מיד בא גבריאל וחבטן בקרקע, ומתו. באותה שעה אמרו: מלך לא דן ולא דנין אותו, לא מעיד ולא מעידין אותו.

Shimon ben Shataḥ turned to his right. The judges forced their faces to the ground out of fear and said nothing. He turned to his left, and they forced their faces to the ground and said nothing. Shimon ben Shataḥ said to them: You are masters of thoughts, enjoying your private thoughts, and not speaking. May the Master of thoughts, God, come and punish you. Immediately, the angel Gabriel came and struck those judges to the ground, and they died. At that moment, when they saw that the Sanhedrin does not have power to force the king to heed its instructions, the rabbis said: A king does not judge others and others do not judge him, and he does not testify and others do not testify concerning him, due to the danger of the matter.

לא חולץ ולא חולצין וכו׳. איני? והאמר רב אשי: אפילו למאן דאמר נשיא שמחל על כבודו ⁠– כבודו מחול, מלך שמחל על כבודו ⁠– אין כבודו מחול, שנאמר: ״שום תשים עליך מלך״, שתהא אימתו עליך! מצוה שאני.

The Mishnah teaches that the king does not perform ḥalitza with his brother’s widow and his brother does not perform ḥalitza with his wife, and R' Yehuda says that he may do so if he wishes. The Talmud challenges R' Yehuda’s opinion: Is that so? But doesn’t Rav Ashi say: Even according to the one who says that with regard to a Nasi who relinquished the honor due him, his honor is relinquished, nevertheless, with regard to a king who relinquished the honor due him, his honor is not relinquished, as it is stated: “You should set a king over you” (Deuteronomy 17:15), meaning that his fear should be upon you. The preservation of a king’s honor is mandated by the Torah. How could R' Yehuda allow him to waive it? The Talmud answers: A mitzva is different; a king is not disgraced if he relinquishes his honor to perform a mitzva.