Narrated Ibn Umar:
I heard Allah’s Messenger (peace be upon him) 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 month of Ramadan as of 30 days.”
(Bukhari Vol. 3, Book 31, Hadith 124)
