أَخْبَرَنَا سُوَيْدُ بْنُ نَصْرٍ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، أَنَّ فَاطِمَةَ بِنْتَ أَبِي حُبَيْشٍ، أَتَتْ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقَالَتْ يَا رَسُولَ اللَّهِ إِنِّي أُسْتَحَاضُ فَلاَ أَطْهُرُ ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّمَا ذَلِكِ عِرْقٌ وَلَيْسَتْ بِالْحِيضَةِ فَإِذَا أَقْبَلَتِ الْحِيضَةُ فَأَمْسِكِي عَنِ الصَّلاَةِ وَإِذَا أَدْبَرَتْ فَاغْسِلِي عَنْكِ الدَّمَ وَصَلِّي ‏"‏ ‏.‏

It was narrated that 'Aishah said: "Fatimah bint Abi Hubaish said to the Messenger of Allah (PBUH): 'O Messenger of Allah (PBUH), I do not become pure. Should I stop praying?' The Messenger of Allah (PBUH) said: 'That is a vein and is not menstruation. When your period comes, stop praying, and when it has passed, then wash the blood from yourself and pray.'" Translation Not Available

Book Ref: Sunan An Nisai Book 3 Hadith 365
Web Ref:  Sunan An Nisai Vol 1 Book 3 Hadith 365