פירוש על עבודה זרה 92:3

Rava answers with a kal vehomer type of argument. The Torah prohibits using the wages of a harlot to offer a sacrifice and it does not matter whether the wages were paid with something attached to the ground or detached. However, the wages of a harlot, if detached from the ground may be used for secular purposes. But an idolatrous object is prohibited even for secular use when it is detached. This makes it more stringent. Therefore, we should also be stringent when it is attached to the ground and not allow it to be used in divine worship. The rocks should be prohibited.