Musnad Ahmad 1156:112

Shuja’ ibn al-Walid narrated to us, from ‘Ali ibn Abi al-A’la, from Abu Sahl, from Massah al-Azdiyyah, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, who said: "The woman experiencing postpartum bleeding would sit [in seclusion] during the time of the Messenger of Allah, may Allah bless him and grant him peace, for forty days. And we used to apply *wars* (a yellow dye) to our faces because of the melasma (dark spots)."