Musannaf Ibn Abi Shayba 3:4834

Abu al-Ahwas narrated to us, from al-Ala’ ibn al-Musayyab, from Ata’, who said: If she becomes pure [from menstruation] at the end of the night, then she should perform the prayer of that night. And if she becomes pure at the end of the day, then she should perform the prayer of that day.