Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 133
Narrated 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."