It is narrated from Sayyiduna Imran bin Husayn (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said to him, or to someone else: "Did you fast the last days of this month?" He said: "No." He (peace and blessings be upon him) said: "When the people stop fasting - or when you stop fasting - fast two days."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15313