Sahih Bukhari Volume 003, Book 031, Hadith Number 184.

Narated By Abu Sa'id : That he had heard the Prophet saying, "Do not fast continuously (practise Al-Wisal), and if you intend to lengthen your fast, then carry it on only till the Suhur (before the following dawn)." The people said to him, "But you practice (Al-Wisal), O Allah's Apostle!" He replied, "I am not similar to you, for during my sleep I have One Who makes me eat and drink."