Sahih Bukhari Volume 003, Book 031, Hadith Number 133.

Narated By Abu Huraira : The Prophet or Abu-l-Qasim said, "Start fasting on seeing the crescent (of Ramadan), and give up fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it), complete thirty days of Sha'ban."