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."

Narrated from Umm al-Mu'mineen Umm Salamah (may Allah be pleased with her) that once the Prophet Muhammad (peace be upon him) took an oath of abstention from his pure wives for a month (i.e., swore to abstain from relations). Then, when twenty-nine days had passed, in the morning or after midday, the Prophet (peace be upon him) went to his wives. Someone said to him, "You had sworn that you would not go to your wives for a month." The Prophet (peace be upon him) replied, "A month can also be of twenty-nine days."