Tosefta su Ester 4:2

Rebbi Yossi Ben Rebbi Yehudah says, “It says [in the verse], ‘… because you cannot come into the gate of the king in sackcloth.’ (Esther 4:2) For sure there are many more Kal Vechomer (a derivation from minor to major) derivations in this case.” And spitting [is forbidden on the Temple Mount] from a Kal Vechomer. Even though a shoe has nothing disgraceful about it, [and still] the Torah says not to go with a shoe [on to the Temple Mount], it is a Kal Vechomer to spitting which is disgraceful, [that it should be for sure forbidden on the Temple Mount].