Hadeeth

Sahih al-Bukhari

Volume 3, Book 31, Number 130:

Narrated 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)."