Is fasting obligatory after the cessation of menstrual bleeding, or after seeing the brown discharge that appears two or three days after its cessation?

If menstrual bleeding stops, even temporarily, the woman must perform ghusl (ritual bathing), pray, and fast, based on the saying of Ibn Abbas: "As for what she sees of purity for an hour, let her perform ghusl and pray." Purity is recognized by the white discharge (al-qassah al-bayda') or dryness of the area. As for the discharges that appear after the cessation of menstruation, if they occur within the usual menstrual days, they are considered menstruation; otherwise, they are not.