Sahih al-Bukhari 1980
Fasting
The Saum (fasting) of Da'wud (David) alayhis salam.
Narrated `Abdullah bin `Amr: Allah's Messenger (ﷺ) 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 Messenger (ﷺ) ! (I can fast more)." He said, "Five?" I replied, "O Allah's Messenger (ﷺ) ! (I can fast more)." He said, "Seven?" I replied, "O Allah's Messenger (ﷺ) ! (I can fast more)." He said, "Nine (days per month)?" I replied, "O Allah's Messenger (ﷺ) ! (I can fast more)" He said, "Eleven (days per month)?" And then the Prophet (ﷺ) said, "There is no fast superior to that of the Prophet (ﷺ) David it was for half of the year. So, fast on alternate days."