Sahih al-Bukhari — Bathing (Ghusl) — Hadith No: 275

Narrated `Aisha:Allah's Messenger (ﷺ) used to fast till one would say that he would never stop fasting, and he would abandon fasting till one would say that he would never fast. I never saw Allah's Messenger (ﷺ) fasting for a whole month except the month of Ramadan, and did not see him fasting in any month more than in the month of Sha'ban

Narrated Ibn `Abbas:The Prophet (ﷺ) never fasted a full month except the month of Ramadan, and he used to fast till one could say, "By Allah, he will never stop fasting," and he would abandon fasting till one would say, "By Allah, he will never fast

Narrated Anas:Allah's Messenger (ﷺ) used to leave fasting in a certain month till we thought that he would not fast in that month, and he used to fast in another month till we thought he would not stop fasting at all in that month. And if one wanted to see him praying at night, one could see him (in that condition), and if one wanted to see him sleeping at night, one could see him (in that condition) too

Narrated `Abdullah bin `Amr bin Al-`As:Allah's Messenger (ﷺ) said to me, "O `Abdullah! Have I not been informed that you fast during the day and offer prayers all the night." `Abdullah replied, "Yes, O Allah's Messenger (ﷺ)!" The Prophet (ﷺ) said, "Don't do that; fast for few days and then give it up for few days, offer prayers and also sleep at night, as your body has a right on you, and your wife has a right on you, and your guest has a right on you. And it is sufficient for you to fast three days in a month, as the reward of a good deed is multiplied ten times, so it will be like fasting throughout the year." I insisted (on fasting) and so I was given a hard instruction. I said, "O Allah's Messenger (ﷺ)! I have power." The Prophet (ﷺ) said, "Fast like the fasting of the Prophet David (ﷺ) and do not fast more than that." I said, "How was the fasting of the Prophet of Allah, David (ﷺ)?" He said, "Half of the year," (i.e. he used to fast on every alternate day). Afterwards when `Abdullah became old, he used to say, "It would have been better for me if I had accepted the permission of the Prophet (which he gave me i.e. to fast only three days a month)