Mishkat al-Masabih #457

Asma’ daughter of Abu Bakr said that a woman asked God's messenger to tell her what one should do when blood from her menses dropped on her garment. He replied, “When blood from her menses drops on the garment of any of you, she should rub it with her fingers and sprinkle water on it, then she may pray in it.” (Bukhari and Muslim.)