Sahih Bukhari Volume 003, Book 031, Hadith Number 201.

Narated By 'Abdullah bin 'Amr : Allah's Apostle was informed about my fasts, and he came to me and I spread for him a leather cushion stuffed with palm fires, but he sat on the ground and the cushion remained between me and him, and then he said, "Isn't it sufficient for you to fast three days a month?" I replied, "O Allah's Apostle! (I can fast more)." He said, "Five?" I replied, "O Allah's Apostle! (I can fast more)." He said, "Seven?" I replied, "O Allah's Apostle! (I can fast more)." He said, "Nine (days per month)?" I replied, "O Allah's Apostle! (I can fast more)" He said, "Eleven (days per month)?" And then the Prophet said, "There is no fast superior to that of the Prophet David it was for half of the year. So, fast on alternate days."