Mishná sur Édouyot 8:9

Ordinary harps are susceptible to impurity, but the harps of Levites are clean. All liquids are susceptible to impurity, but the liquids in the Temple slaughtering house are clean. All scrolls convey impurity to the hands, excepting the scroll of the Temple courtyard. A wooden toy horse is clean. The belly-lute, the donkey-shaped musical instrument and the erus are susceptible to impurity. Rabbi Judah says: the erus is susceptible to sitting impurity since the wailing woman sits on it. A weasel-trap is susceptible to impurity, but a mouse- trap is clean.