Silsilat al-Ahadith al-Sahihah

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Count the days of Sha'ban for Ramadan (i.e., keep track of the dates by the moon) and do not join it with Ramadan (by fasting an extra day), except if a man is in the habit of fasting on a particular day and it coincides with that day (then he may fast). Begin fasting by sighting the moon and break the fast by sighting it. If (on the evening of the 29th) the moon is hidden by clouds or mist, then the counting is not concealed (i.e., complete thirty days based on the count)."