Sahih Bukhari
Sahih Bukhari Volume 003, Book 031, Hadith Number 131.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 31. Fasting
Narated By Abdullah bin Umar : Allah's Apostle said, "The month (can be) 29 nights (i.e. days), and do not fast till you see the moon, and if the sky is overcast, then complete Sha'ban as thirty days."