משנה אויף במדבר 5:16

If she said, “I am defiled to you”, she gives him a receipt for her ketubah and goes out [with a get]. But if she says, “I am pure”, they bring her up to the east gate, Nicanor’s gate, where they give women suspected of adultery the water to drink, purify women after childbirth and purify lepers. A priest seizes her clothing if they are torn, then they are torn, and if they become unstitched, then they are unstitched, until he uncovers her bosom, and he undoes [the braids of] her hair. Rabbi Judah says: if her bosom was beautiful he does not uncover it, and if her hair was beautiful he does not undo it.