Sahih Muslim Book 6, Hadith Number 2376.

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(may Allah be pleased with both of them)reported Allah’s Apostle (may peace be upon him) as saying: We are an unlettered people who can neither write nor count. The month is thus, and thus, folding his thumb when he said it the third time. This Hadith has been narrated on the authority of Aswad b. Qais with the same chain of transmitters, but herein no mention has been made of the other month (consisting of) thirty days.