Sahih Muslim Book 6, Hadith Number 2367.
Chapter : Fasting in Ramadan should necessarily be commenced with the sight of the new moon and finished with the sight of the new moon. If the weather is cloudy at the beginning or at the end, then complete thirty days.
Ibn’Umar (Allah be pleased with-both of them) reported Allah’s Messenger (may peace be upon him) as saying: The month of Ramadan may consist of twenty-nine days. So do not fast till you have sighted it (the new moon) and do not breakfast, till you have sighted it (the new moon of Shawwal), and if the sky is cloudy for you, then calculate.