Sahih Muslim Book 6, Hadith Number 2372.

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 may consist of twenty-nine days.