Musnad Abu Ya'la al-Mawsili 209:137

Abu Khaythama narrated to us, Muhammad ibn Ubayd narrated to us, Muhammad ibn Amr narrated to us, from Abu Salamah, who said: Umm Salamah said: "I was with the Messenger of Allah, peace and blessings of Allah be upon him, under his blanket, and I experienced what women experience. So, I slipped out from under the blanket." She said: "The Messenger of Allah, peace and blessings of Allah be upon him, said: 'Have you menstruated?' I said: 'Yes.' He said: 'That is what has been decreed for the daughters of Adam.'" "So I tidied myself up, then I returned. He said: 'Come and enter with me.' She said: 'So I entered with him under the blanket.'"