Sahih Bukhari
Sahih Bukhari Volume 003, Book 031, Hadith Number 138.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 31. Fasting
Narated By Abu Huraira : The Prophet said, "None of you should fast a day or two before the month of Ramadan unless he has the habit of fasting (Nawafil) (and if his fasting coincides with that day) then he can fast that day."