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]. (DE)
App reference (DE): Book 9, Hadith 271 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1261 (DE)