Sahih Bukhari Volume 008, Book 074, Hadith Number 294.

Narated By Abdullah bin 'Amr : The news of my fasting was mentioned to the Prophet. So he entered upon me and I put for him a leather cushion stuffed with palm-fibres. The Prophet sat on the floor and the cushion was between me and him. He said to me, "Isn't it sufficient for you (that you fast) three days a month?" I said, "O Allah's Apostle! (I can fast more than this)." He said, "You may fast) five days a month." I said, "O Allah's Apostle! (I can fast more than this)." He said, "(You may fast) seven days." I said, "O Allah's Apostle!" He said, "Nine." I said, "O Allah's Apostle!" He said, "Eleven." I said, "O Allah's Apostle!" He said, "No fasting is superior to the fasting of (the Prophet David) which was one half of a year, and he used, to fast on alternate days. (See Hadith No. 300, Vol 3)