Musa bin Talhah (may Allah have mercy on him) narrates: I heard Sayyiduna Abu Dharr (may Allah be pleased with him) at al-Rabadhah saying that the Messenger of Allah (peace and blessings be upon him) said: "O Abu Dharr, when you fast three days of the month, then (fast the 13th, 14th and 15th)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8445