Sahih Bukhari

Narrated `Abdullah bin `Amr: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 David (ﷺ)) which was one half of a year, and he used, to fast on alternate days. (See Hadith No. 300, Vol)