Purification (Kitab Al-Taharah)
Dawud :: Book 1 : Hadith 311
Narrated Umm Salamah, Ummul Mu'minin:
The woman having bleeding after delivery (puerperal haemorrhage) would refrain (from prayer) for forty days or forty nights; and we would anoint our faces with an aromatic herb called wars to remove dark spots.