Ahmad ibn Muhammad narrated from Ibn Mahbub, from Al-Fadl ibn Yunus, who said: I asked Abu Al-Hasan Al-Awwal عليه السلام, and said: “A woman sees purity (from menstruation) before sunset - what should she do regarding prayer?” Imam عليه السلام replied: "If she sees purity after four footsteps have passed from the time of midday (after zawal), then she only performs the ‘Asr prayer, because the time for Dhuhr entered upon her while she was in menstruation and exited while she was still in menstruation. Therefore, it is not obligatory for her to pray Dhuhr, as Allah سبحانه وتعالى has exempted her from prayer while she was in menstruation, and that exemption is greater." Imam عليه السلام continued: "However, if the woman sees blood after four footsteps have passed from the time of midday, she should refrain from prayer. Then, if she becomes pure from the blood later, she must make up the Dhuhr prayer because the time for Dhuhr entered upon her while she was pure and exited while she was pure. Therefore, she has missed the Dhuhr prayer, and it becomes obligatory for her to make it up."