Musnad Ahmad 1156:90

Abu al-Nadr narrated to us, he said: Abu Khaythama—meaning Zuhayr ibn Mu'awiyah—narrated to us, from Ali ibn Abd al-A'la, from Abu Sahl, from the people of Basra, from Massa, from Umm Salamah, who said: "The woman experiencing postpartum bleeding during the time of the Messenger of Allah, may Allah bless him and grant him peace, would sit [in a state of impurity] after her postpartum bleeding for forty days," or forty nights—Abu Khaythama was in doubt—"and we would apply *wars* (a yellow dye) to our faces because of the discoloration."