It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her); she said: In the time of the Messenger of Allah (peace and blessings be upon him), women would sit for forty nights after childbirth, and we would pray in that state while our faces were smeared with wars (saffron-like dye) because of dark blemishes.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20180