Volume 3, Book 31, Number 124:
Narrated Ibn Umar:
I heard Allah’s Apostle saying, “When you see the crescent (of the month of Ramadan), start fast –
ing, 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)”.