It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Fasting on the thirteenth, fourteenth and fifteenth (of the month) is equal to fasting the whole year and breaking the fast."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10194