It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Did you fast any of the last days of this month?" I said: "No, by Allah, O Messenger of Allah." He (peace and blessings be upon him) said: "Then when you have completed (the fasts of) Ramadan and broken your fast, fast two days in place of that day."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15311