Sayyidah Maymunah bint Sa'd (may Allah be pleased with her) said: O Messenger of Allah, give us a ruling about fasting. He (peace and blessings be upon him) said: "Three days of every month; whoever is able, let him fast them, for each day wipes out ten sins, and it purifies from sin just as water cleans a garment."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21523