It is narrated from Sayyidah Asma bint Abi Bakr (may Allah be pleased with her) that a woman asked the Prophet (peace and blessings be upon him) what one of us should do if menstrual blood gets on her garment. 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 / 20666