Sahih Muslim Book 6, Hadith Number 2371.

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 is thus and thus and thus (i.e. pointing with his fingers thrice), and he held back his thumb at the third time (in order to show that it can also consist of twenty-nine days).