Sahih Bukhari Volume 003, Book 031, Hadith Number 124.

Narated By Ibn Umar : I heard Allah's Apostle saying, "When you see the crescent (of the month of Ramadan), start fasting, and when you see the crescent (of the month of Shawwal), stop fasting; and if the sky is overcast (and you can't see It) then regard the crescent (month) of Ramadan (as of 30 days)".

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