Parashas Shemos
וַיֹּ֖אמֶר אַל־תִּקְרַ֣ב הֲלֹ֑ם שַׁל־נְעָלֶ֙יךָ֙ מֵעַ֣ל רַגְלֶ֔יךָ כִּ֣י הַמָּק֗וֹם אֲשֶׁ֤ר אַתָּה֙ עוֹמֵ֣ד עָלָ֔יו אַדְמַת־קֹ֖דֶשׁ הֽוּא׃
And [GodcGod See note at v. 4.] said, “Do not come closer! Remove your sandals from your feet, for the place on which you stand is holy ground!”
Torah Temimah points to TT / Berachos (verified).
של נעליך — מכאן אמרו: אסור לרקוק בהר הבית, קל וחומר ממנעל.
English: “Remove your shoes” — from here they said: It is forbidden to spit on the Temple Mount, a fortiori from a shoe.
Yishmael # (kal vachomer)
Chazal derive from the command to remove shoes both the specific rule for the Temple Mount and the a-fortiori ban on spitting there.