It was narrated from 'Aishah that the daughter of Abu Hubaish said: "O Messenger of Allah, I do not become pure, so should I stop praying? He said: "no, that is a vein." - (One of the narrators) Khalid said, in what I read from him - "and it is not menstruation, so when your period comes, stop praying, and when it goes, wash the blood from yourself and pray."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا أَبُو الأَشْعَثِ، قَالَ حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، قَالَ سَمِعْتُ هِشَامًا، يُحَدِّثُ عَنْ أَبِيهِ، عَنْ عَائِشَةَ، أَنَّ بِنْتَ أَبِي حُبَيْشٍ، قَالَتْ يَا رَسُولَ اللَّهِ إِنِّي لاَ أَطْهُرُ أَفَأَتْرُكُ الصَّلاَةَ قَالَ " لاَ إِنَّمَا هُوَ عِرْقٌ " . قَالَ خَالِدٌ وَفِيمَا قَرَأْتُ عَلَيْهِ " وَلَيْسَتْ بِالْحِيضَةِ فَإِذَا أَقْبَلَتِ الْحِيضَةُ فَدَعِي الصَّلاَةَ وَإِذَا أَدْبَرَتْ فَاغْسِلِي عَنْكِ الدَّمَ ثُمَّ صَلِّي " .
sunnah.com