Sahih Muslim Book 006, Hadith Number 2370.

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 nights. So do not fast till you have sighted it (the new moon) and do not break it till you have sighted it, except when the sky is cloudy for you, and if it is so, then calculate it.

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