Sahih Muslim Book 6, Hadith Number 2373.

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.

Abdullah b. ‘Umar (Allah be pleased with both of them) reported Allah’s Messenger (may peace be upon him) as saying: The mouth (of Ramadan) is thus and thus, and thus i.e. ten, ten and-nine.