Musannaf Ibn Abi Shayba 1:801

Ghundar narrated to us, from Shu'bah, who said: I asked Hammad about the woman [and her hair]. He said: "If she sees that the water has reached it, it suffices her; and if she sees that the water has not reached it, then she should undo it." And al-Hakam said: "She should moisten its roots and its ends, and she should not undo it."