Said Rabbi Ishmael: Thereupon I took a piece of woolen cloth and gave it to Rabbi Akiba, and Rabbi Akiba gave it to a servant of ours, saying, “Go and lay this cloth beside a woman who immersed herself and yet did not become pure, and let her immerse [herself a second time]. For if that woman will come and will declare the measure of her menstrual flow before the company, it is known that one forbids [her to her husband] and the majority permit. Say to that woman, ‘Touch this cloth with the end of the middle finger of thy hand, and press not the end of thy finger upon it, but [only] as a man [presseth] who taketh a hair, which hath fallen therein, from out his eye-ball, pushing it very gently.’”
Ishmael and the Angels of Akiba
Heikhalot Rabbati 20:2