Sahih Al-Bukhari – Volume 3 – Book 30 – P.86

1907. Narrated ‘Abdullãh bin ‘Umar : Allah’s Messenger, peace be upon him, said, “The month (can be) 29 nights (i.e., days), and do not fast till you see the crescent-moon, and if the sky is overcast, then complete Sha’ban as thirty days.”