Sahih Bukhari

Narrated `Abdullah bin `Amr bin Al-`As:""Once Allah's Messenger (ﷺ) came to me,"" and then he narrated the whole narration, i.e. your guest has a right on you, and your wife has a right on you. I then asked about the fasting of David. The Prophet (ﷺ) replied, ""Half of the year,"" (i.e. he used to fast on every alternate day)