Sahih Bukhari: Hadith 6055

The news of my fasting was mentioned to the Prophet (ﷺ) . So he entered upon me and I put for him a leather cushion stuffed with palm-fibres. The Prophet (ﷺ) 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 Messenger (ﷺ)! (I can fast more than this)." He said, "You may fast) five days a month." I said, "O Allah's Messenger (ﷺ)! (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 Messenger (ﷺ)!" He said, "Eleven." I said, "O Allah's Messenger (ﷺ)!" He said, "No fasting is superior to the fasting of (the Prophet (ﷺ) David) which was one half of a year, and he used, to fast on alternate days. (See Hadith No. 300, Vol 3)