It was narrated that 'Aishah said:Fatimah bint Abi Hubaish said to the Messenger of Allah (ﷺ): "O Messenger of Allah (ﷺ), I do not become pure. Should I stop praying?" The Messenger of Allah (ﷺ) said: "That is a vein and is not menstruation. When your period comes, stop praying, and when the same amount of time as your regular period has passed, then wash the blood from yourself and pray."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ قَالَتْ فَاطِمَةُ بِنْتُ أَبِي حُبَيْشٍ لِرَسُولِ اللَّهِ صلى الله عليه وسلم لاَ أَطْهُرُ أَفَأَدَعُ الصَّلاَةَ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّمَا ذَلِكِ عِرْقٌ وَلَيْسَتْ بِالْحِيضَةِ فَإِذَا أَقْبَلَتِ الْحِيضَةُ فَدَعِي الصَّلاَةَ وَإِذَا ذَهَبَ قَدْرُهَا فَاغْسِلِي عَنْكِ الدَّمَ وَصَلِّي " .
Nesai
The Book of Menstruation and Istihadah • Hadith 366