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 a garment that has menstrual blood on it, and he (peace and blessings be upon him) said: "Let her scrape it off, then rub it with water, then pray in it."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20660