Mu'adhah Al-'Adawiyah (May Allah be pleased with her) reported:
I asked 'Aishah (May Allah be pleased with her). Did the Messenger of Allah (ﷺ) use to observe three days of Saum (fasting) in every month?" She replied, "Yes." I asked, "On which days in the month did he observed fast?" She replied that he did not mind on which days of the month he observed fast.[Muslim].
App reference: Book 9, Hadith 271
Arabic/English book reference: Book 9, Hadith 1261