Sayyidah Asma' bint Abi Bakr (may Allah be pleased with her) said: I heard the Messenger of Allah (peace and blessings be upon him) when a woman asked him (peace and blessings be upon him) about menstrual blood that gets on her clothes, and he (peace and blessings be upon him) said: "Wash it with water, then sprinkle water over the rest of your garment, and pray in it."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20721