Sahih Muslim
Sahih Muslim Book 006, Hadith Number 2368.
Hadith Section/Book: Sahih Muslim Sahih Muslim Book 06. Fasting
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 month (of Ramadan) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and 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 month (of Ramadan) may consist of twenty nine days; so when you see the new moon observe fast and when you see (the new moon again at the commencement of the month of Shawwal) then break It, and if the sky is cloudy for you, then calculate it (and complete thirty days).