Musannaf Ibn Abi Shayba 3:4832

Ibn Fudayl narrated to us, from Layth, from Ata and Tawus, that they both said: If a menstruating woman becomes pure before the setting of the sun, she performs ghusl and prays Dhuhr and Asr. And if she becomes pure before the dawn, she performs ghusl and prays Maghrib and Isha.