Ahmad ibn Muhammad narrated from Ibn Mahbub, from Abu Ayyub Al-Khazzaz, from Muhammad ibn Muslim:
Abu Ja’far عليه السلام said: "For a menstruating woman, it is sufficient if the moisture of the water reaches her hair."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 423