Sahih Bukhari Volume 3, Book 30, Hadith 1975

حَدَّثَنَا إِسْحَاقُ، أَخْبَرَنَا هَارُونُ بْنُ إِسْمَاعِيلَ، حَدَّثَنَا عَلِيٌّ، حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنِي أَبُو سَلَمَةَ، قَالَ حَدَّثَنِي عَبْدُ اللَّهِ بْنُ عَمْرِو بْنِ الْعَاصِ رضى الله عنهما قَالَ دَخَلَ عَلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم فَذَكَرَ الْحَدِيثَ يَعْنِي ‏"‏ إِنَّ لِزَوْرِكَ عَلَيْكَ حَقًّا، وَإِنَّ لِزَوْجِكَ عَلَيْكَ حَقًّا ‏"‏‏.‏ فَقُلْتُ وَمَا صَوْمُ دَاوُدَ قَالَ ‏"‏ نِصْفُ الدَّهْرِ ‏"‏‏

Narrated 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).