Sahih Muslim
Sahih Muslim Book 006, Hadith Number 2364.
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.
Ibn Umar reported that Allah's Messenger (may peace be upon him) made a mention of Ramadan and he with the gesture of his hand said: The month is thus and thus. (He then withdrew his thumb at the third time). He then said: Fast when you see it, and break your fast when you see it, and if the weather is cloudy calculate it (the months of Sha'ban and Shawwal) as thirty days.
Ibn Umar reported that Allah's Messenger (may peace be upon him) made a mention of Ramadan and he with the gesture of his hand said: The month is thus and thus. (He then withdrew his thumb at the third time). He then said: Fast when you see it, and break your fast when you see it, and if the weather is cloudy calculate it (the months of Sha'ban and Shawwal) as thirty days.