Musnad Ahmad 1156:120

Hassan ibn Musa narrated to us, Zuhayr narrated to us, Abu al-Hasan al-Ahwal—meaning Ali ibn Abd al-A’la—narrated to us, from Abu Sahl, from Mussah, from Umm Salamah, who said: "The woman experiencing postpartum bleeding during the time of the Messenger of Allah (peace and blessings of Allah be upon him) would sit [in seclusion] after her postpartum bleeding for forty days or forty nights. And we would apply *wars* (a yellow dye) to our faces because of the melasma (dark spots)."