Fasting

Bukhari :: Book 3 :: Volume 31 :: Hadith 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."