It is narrated from Ahmad ibn Muhammad ibn Khalid, 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, "How long should she wait?" 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)."