Mu'adhah (may Allah have mercy on her) said: I asked Sayyidah Aishah (may Allah be pleased with her) about blood on clothes. And it is reported from Bishr (may Allah have mercy on him) that she said: I asked: What is your view if, even after cleaning the blood from the clothes, the traces do not go away? Sayyidah Aishah (may Allah be pleased with her) said: Water is a means of purification (or: water is pure).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4114