Reference for Kelim 23:3

מַה בֵּין מֶרְכָּב לְמוֹשָׁב. מֶרְכָּב חָלַק מַגָּעוֹ מִמַּשָּׂאוֹ, וּמוֹשָׁב לֹא חָלַק מַגָּעוֹ מִמַּשָּׂאוֹ. תָּפִית שֶׁל חֲמוֹר שֶׁהוּא יוֹשֵׁב עָלֶיהָ, טָהוֹר. שִׁנָּה בָהּ אֶת הַנְּקָבִין, אוֹ שֶׁפֵּרְצָן זֶה לְתוֹךְ זֶה, טְמֵאָה:

What is the practical difference between [the uncleanness of an object used for] riding upon and [one used for] sitting upon? [An object] ridden upon has a distinction between contact with it and carrying it, but in the case of [an object that is] sat upon, there is no difference between contact with it and carrying it. The pack-frame of a donkey on which he sat is impure, but if he changed the size of the spaces or if they have been broken one into another, it is susceptible to impurity.

Jastrow