Sayyiduna Qatadah bin Milhan al-Qaysi (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) used to order us to fast the White Days, that is, the 13th, 14th and 15th (of the month), and he would say: "This is (like) fasting the whole of time."
Imam al-Tabarani (may Allah have mercy on him) said: The wording of (all these narrators) is the same.
Hadith Referenceالمعجم الكبير للطبراني / باب القاف / 16073