Miszna do Sanhedryn 2:7

The beam of a wool-combers’ balance is susceptible to impurity on account of the hooks. And that of a householder, if it has hooks is also susceptible to impurity. The hooks of porters are clean but those of peddlers are susceptible to impurity. Rabbi Judah says: in the case of the peddlers' [hooks], [the hook] that is in front is susceptible to impurity but that which is behind is clean. The hook of a couch is susceptible to impurity but that of bed poles is clean. [The hook of] a chest is susceptible to impurity but that of a fish trap is clean. That of a table is susceptible to impurity but that of a wooden candlestick is clean. This is the general rule: any hook that is attached to a susceptible vessel is susceptible to impurity, but one that is attached to a vessel that is not susceptible to impurity is clean. All these, however, are by themselves clean.