Mukhtasar Bukhari

Narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is (at least) twenty-nine days, so do not fast until you see the crescent. Then if the sky is overcast (i.e., clouds cover the sky and you cannot see the crescent), complete the count of thirty days."