Musannaf Ibn Abi Shayba 1:1007

1007. 'Abd al-A'la ibn 'Abd al-A'la narrated to us, from Muhammad ibn Ishaq, from Fatimah bint al-Mundhir, from Asma' bint Abi Bakr, who said: We were in her care along with the daughters of her daughter. One of us would become pure, then pray, then she would be afflicted with a slight yellowish discharge. She would ask her, and she would say: "Withdraw from prayer as long as you see that, until you see nothing but pure whiteness."