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: "Indeed, Allah Almighty has made the moon a means for determining the times. Therefore, when you see it, begin fasting, and when you see it (again), stop fasting. But if clouds cover you (and you cannot see the moon), then complete the count (of days), and know well that the month does not exceed thirty days."