Sahih Muslim Book 6, Hadith Number 2374.

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, and he flapped his hands with all their fingers twice, but at the third turn, folded his right thumb or left thumb (in order to give an idea of twenty-nine).