Talhah bin Yahya (may Allah have mercy on him) narrates the like of it from Musa (may Allah have mercy on him), that he (peace and blessings be upon him) said: "Then keep these three fasts on the 13th, 14th and 15th of the month."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19400