The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, narrated to me from Ahmad ibn Muhammad, from his father, from Sa’d ibn Abdullah, from Ahmad ibn Muhammad, from Muhammad ibn Khalid Al-Barqi, from Muhammad ibn Amr ibn Sa’id, from Abu Al-Hasan Al-Ridha عليه السلام, who said: I asked Imam عليه السلام about a menstruating woman and the duration of her sitting (abstaining from acts of worship). Imam عليه السلام said: "She should wait for the duration of her usual menstruation, then add three days as a precaution. After that, she is considered to be in the state of istihadha (irregular bleeding)."