Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "The month is only twenty-nine days. So do not fast until you see the moon, and do not break the fast until you see the moon. If it is clouded over for you, then estimate (the days) for it."
Nafi' says: The moon would be looked for on behalf of Sayyiduna Ibn Umar (may Allah be pleased with them both) when twenty-nine days of Sha'ban had passed. If it was seen, he would fast; if it was not seen and nothing obstructed the view of it, he would be in a state of not fasting. If clouds or the like obstructed the view of the moon, he would begin the morning fasting. And he would end the fast together with the people and would not rely on any calculation.
Ibn Awn says: I mentioned the practice of Sayyiduna Ibn Umar (may Allah be pleased with them both) to Muhammad bin Sirin, and he did not like it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7923