Riyad as-Salihin
Riyad as-Salihin 1262
The Desirability of Observing Three Days of Fasting in Every Month
Arabic hadith text
وعن أبي ذر رضي الله عنه، قال: قال رسول الله صلى الله عليه وسلم: "إذا صمت من الشهر ثلاثًا، فصم ثلاث عشرة، وأربع عشرة وخمس عشرة" رواه الترمذي وقال: حديث حسن.
Translation
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].