Abu Dharr (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "If you want to observe Saum on three days in a month, then fast on the thirteenth, fourteenth and fifteenth of the (lunar) month."[At-Tirmidhi]. (IT)
App reference (IT): Book 9, Hadith 272 (IT)
Arabic/English book reference (IT): Book 9, Hadith 1262 (IT)