Keilim 26:7

If no work on a utensil is lacking, then intention alone renders it susceptible to impurity, but if some work is lacking, then intention alone does not render it susceptible to impurity except for a hide for riding upon.

Keilim 26:8

Hides belonging to a homeowner are rendered susceptible to impurity through intention, while those belonging to a tanner are not rendered susceptible through intention. Hides taken stealthily by a thief are rendered susceptible by intention (because the owner has despaired of getting them back), but those taken forcefully by a robber are not rendered susceptible by intention (because the owner still hopes to recover them). Rabbi Shimon says it’s actually the opposite: those taken by a robber are rendered susceptible by intention (because the owner despairs of wresting them from the robber) while those taken by a thief are not rendered susceptible by intention because the owner doesn’t abandon his hope that he will get them back.