(a) It is narrated from al-Hasan (may Allah have mercy on him) that when a woman sees a slight trace (of discharge), she should refrain from prayer, for it is menstruation.
(b) It is narrated from Abu Salamah (may Allah have mercy on him) that when a woman sees a slight trace (of discharge), she should look to the days in which she used to menstruate, and refrain from prayer only in those days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1594