Fatimah daughter of Abu Hubaish came to the Messenger of Allah ﷺ and said: I am a woman who has prolonged flow of blood; I am never purified ; should I abandon prayer ? He replied: This is (due to) a vein, and not menstruation. When the menstruation begins, you should abandon prayer ; when it is finished, you should wash away the blood and pray.  

أَنَّ فَاطِمَةَ بِنْتَ أَبِي حُبَيْشٍ جَاءَتْ رَسُولَ اللَّهِ ﷺ فَقَالَتْ إِنِّي امْرَأَةٌ أُسْتَحَاضُ فَلاَ أَطْهُرُ أَفَأَدَعُ الصَّلاَةَ قَالَ إِنَّمَا ذَلِكِ عِرْقٌ وَلَيْسَتْ بِالْحَيْضَةِ فَإِذَا أَقْبَلَتِ الْحَيْضَةُ فَدَعِي الصَّلاَةَ وَإِذَا أَدْبَرَتْ فَاغْسِلِي عَنْكِ الدَّمَ ثُمَّ صَلِّي  

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