Mukhtasar Muslim

It is narrated from Sayyiduna Imran bin Husain (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to him or to someone else: "Did you fast any days at the beginning of Sha'ban?" He replied, "No." So the Prophet (peace and blessings be upon him) said, "When you have broken your fast, then fast for two days." (That is, when you have finished the month of Ramadan, then fast for two days.)