Sahih Bukhari Volume 003, Book 031, Hadith Number 130.

Narated By Abdullah bin Umar : Allah's Apostle mentioned Ramadan and said, "Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha'ban as 30 days)."