Musnad Ahmad 1167:9

Abu Mu'awiyah narrated to us, he said: Hisham ibn 'Urwah narrated to us, from Fatimah bint al-Mundhir, from Asma' bint Abi Bakr, she said: A woman came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, what if a woman gets some of her menstrual blood on her garment?" The Messenger of Allah, may Allah bless him and grant him peace, said: "She shall scrape it, then rub it with water, then pray in it."