Sayyiduna Imran bin Husayn (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said to him or to another person: "Have you fasted at the end of Sha'ban?" He said: No. He (peace and blessings be upon him) said: "When you break your fast, then fast two days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7969