Mu'adhah al-Adawiyyah (may Allah have mercy on her) narrates that she asked Sayyidah Aishah (may Allah be pleased with her): "Did the Messenger of Allah (peace and blessings be upon him) use to fast three days of every month?" She said: "Yes." I said: "On which days of the month did he fast?" She said: "He did not mind on which days of the month he fasted."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8448