A woman named Mu’adhah narrates: I asked Sayyidah Aisha (may Allah be pleased with her): Did the Prophet Muhammad (peace be upon him) observe three fasts every month? She replied: Yes. I asked: On which days? She said: The Prophet Muhammad (peace be upon him) did not mind on which days he would fast.
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3654
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (2117): م.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط الشيخين