Quotation על שבת 186:4

If he sat on a bed and there were four cloaks under the four legs of the bed, all become unclean, since the bed cannot stand on three legs; But Rabbi Shimon declares them clean. If he rode on a beast and there were four cloaks under the legs of the beast, they are clean, since the beast can stand upon three legs. If there was one cloak under its two forelegs or its two hindlegs, or under a foreleg and a hindleg, it becomes unclean. Rabbi Yose says: a horse conveys uncleanness through its hindlegs and a donkey through its forelegs, since a horse leans upon its hindlegs and a donkey upon its forelegs. If he sat on a beam of an olive-press, the vessels in the olive-press receptacle are unclean. [If he sat] on a clothing press, the garments beneath it are clean. Rabbi Nehemiah declares them unclean.