Sahih Muslim Book 6, Hadith Number 2377.

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.

Sa’d b. ‘Ubaida reported that Ibn ‘Umar(Allah be pleased with both of them) heard a person saying: This night is the midnight (of the month). Upon this he said to him: How do you know that it is the midnight (of the month), for I heard the Messenger of Allah (may peace be upon him) as saying: The month is thus and thus (and he pointed with his ten fingers twice) and thus (i.e. at the third time he pointed with all his fingers but withdrew or folded his thumb)?