Sahih Bukhari Volume 3, Book 31, Hadith Number 195.

Narated By ‘Abdullah bin ‘Amr bin Al-‘As : “Once Allah’s Apostle 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 replied, “Half of the year,” (i.e. he used to fast on every alternate day).