If a person forgetfully or neglectfully performs tayammum with a usurped thing, his tayammum is valid. However, if he has usurped that thing and he forgets that he has usurped it, then based on obligatory precaution, his tayammum is invalid.
Ruling 686.* If a person is imprisoned in a usurped place and both the water and the soil of that place are usurped, he must perform prayers with tayammum; however, when he performs tayammum, he must not strike his hands on the ground; rather, he must suffice with placing them on the ground.