Musannaf Ibn Abi Shayba 3:6489

Humaid narrated to us, from al-Hasan ibn Salih, from Ash'ath, from al-Hasan, from Uthman ibn Abi al-'As, who said: "If a woman sees yellow discharge during days other than her menstruation, if it exceeds her menstrual days by one day or two, she counts it as part of her menstruation. If it exceeds two days, then she is a *mustahada* (a woman experiencing irregular vaginal bleeding). If she usually menstruates for six days and sees blood for eight days, she counts it as part of her menstruation. If she sees it for more than eight days, then she is a *mustahada*. And Allah knows best."