Musannaf Ibn Abi Shayba 3:6488

Yahya ibn Adam narrated to us, he said: Ibrahim ibn al-Zabraqan narrated to us, from al-Shaybani, from Musa ibn Abi Kathir, from Tawus, who said: "If a woman exceeds [the duration of] her menstruation, she should perform ghusl (ritual bath)." And Hammad said regarding a woman who exceeds the days of her menstruation: "She should not perform ghusl, for a woman might do that [out of confusion or error]."