It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The month is sometimes twenty-nine days, so if clouds cover (the sky) for you, then estimate it." Nafi' says that Sayyiduna Umar (may Allah be pleased with him), if after the 29th of Sha'ban there were clouds or dust, would fast.