Sahih al-Bukhari 1974
Sahih al-Bukhari · Book 30: Fasting · The right of the guest in fasting.
Narrated `Abdullah bin `Amr bin Al-`As:
حَدَّثَنَا إِسْحَاقُ ، أَخْبَرَنَا هَارُونُ بْنُ إِسْمَاعِيلَ ، حَدَّثَنَا عَلِيٌّ ، حَدَّثَنَا يَحْيَى ، قَالَ حَدَّثَنِي أَبُو سَلَمَةَ ، قَالَ حَدَّثَنِي عَبْدُ اللَّهِ بْنُ عَمْرِو بْنِ الْعَاصِ ـ رضى الله عنهما ـ 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 ".
"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).