When the menstruation begins, you should abandon prayer; when the period equal to its length of time passes, you should wash away the blood and pray.
وَقَالَ فَإِذَا أَقْبَلَتِ الْحَيْضَةُ فَاتْرُكِي الصَّلاَةَ فَإِذَا ذَهَبَ قَدْرُهَا فَاغْسِلِي الدَّمَ عَنْكِ وَصَلِّي