Muhammad ibn Ali ibn Mahbub narrated from Ya'qub ibn Yazid, from Muhammad ibn Al-Fudayl, who said:
I asked Abu Al-Hasan عليه السلام about how much water is sufficient for a menstruating woman (to perform ghusl). Imam عليه السلام said: 'A faraq.'
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 423