Reference for Leviticus 15:4

A bride’s stool from which the covering-boards have been taken: Beth Shammai pronounces it [liable to become] unclean, And Beth Hillel pronounce it not [liable to become] unclean. Shammai says: “Even the framework of a stool [by itself is liable to become] unclean.” A stool which has been set in a baker’s trough: Beth Shammai pronounces it [liable to become] unclean, And Beth Hillel pronounces it not [liable to become] unclean. Shammai says: “Even one made therein [is liable to become unclean].”