Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 195
Narrated '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).