Qatadah bin Milhan (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) directed us to observe Saum (fasting) on the thirteenth, fourteenth and fifteenth of the (lunar) month.[Abu Dawud]. (TH)
App reference (TH): Book 9, Hadith 273 (TH)
Arabic/English book reference (TH): Book 9, Hadith 1263 (TH)