Is prayer obligatory when a drop of menstrual blood appears at the beginning of the day, then stops for hours before its usual flow?

If a woman's menstruation ceases for a few hours (such as three or four hours), she refrains from prayer during this time. However, if this dryness continues for a day or two, scholars have differed on whether this period of purity is considered part of the menstruation or if it is a state of purity during which the woman is obligated to pray.