It is narrated from Sayyiduna Jarir bin Abdullah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever fasts three days of the month, let him fast the White Days (Ayyam al-Bid): the 13th, 14th and 15th."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 2378