Sahih Bukhari
Sahih Bukhari Volume 003, Book 031, Hadith Number 124.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 31. Fasting
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)".