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 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)."
Imam al-Bukhari (may Allah have mercy on him) recorded the same from Malik (may Allah have mercy on him), except that he said: Complete the count of thirty days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7925