Sahih Muslim Book 6, Hadith Number 2375.

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, and Shu’ba (one of the narrators) (gave a practical demonstration how the Holy prophet (may peace be upon him) explained to them) by unfolding his hands thrice and folding his thumb at the third turn. ‘Uqba (one of the narrators in this chain of transmitters) said: I think that he said that the month consists of thirty days and unfolded his palm three times.