Sahih Bukhari Volume 003, Book 031, Hadith Number 145.

Narated By 'Abdullah : The Prophet fasted for days continuously; the people also did the same but it was difficult for them. So, the Prophet forbade them (to fast continuously for more than one day). They slid, "But you fast without break (no food was taken in the evening or in the morning)." The Prophet replied, "I am not like you, for I am provided with food and drink (by Allah)."