Hadith 10:1577
Abu Bakr narrated to us, he said: Hafs narrated to us, from Ash'ath, from 'Ata', who said: The maximum [time] that a menstruating woman sits [i.e., refrains from prayer] is fifteen nights.
حدثنا أبو بكر قال نا حفص عن اشعث عن عطاء قال أقصى ما تجلس الحائض خمس عشرة ليلة