Musannaf Ibn Abi Shayba 3:4835

Waki’ narrated to us, from Sufyan, from Mansur, from al-Hakam, who said: If she sees the purity (the end of menstruation) before Maghrib, she prays Dhuhr and ‘Asr. And if she sees it before Fajr, she prays Maghrib and ‘Isha.