Sayyidah Asma bint Abi Bakr (may Allah be pleased with her) said: The Messenger of Allah (peace and blessings be upon him) was asked about menstrual blood that gets on a garment, and he (peace and blessings be upon him) said: "Scrape it off, then rub it with water, then sprinkle it, and pray in it."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20665