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 do not fast until you see the crescent, and do not celebrate Eid until you see it. If clouds cover you, then estimate." Nafi' says that for this reason, Sayyiduna Ibn Umar (may Allah be pleased with them both) used to send someone to look for the crescent after the twenty-ninth of Sha'ban. If the crescent was sighted, it was fine; if it was not sighted and there were no clouds or dust, then he would not fast the next day. But if there were clouds or dust, he would fast.