(35) بَاب النَّهْيِ عَنْ صَوْمِ الدَّهْرِ لِمَنْ تَضَرَّرَ بِهِ أَوْ فَوَّتَ بِهِ حَقًّا أَوْ لَمْ يُفْطِرْ الْعِيدَيْنِ وَالتَّشْرِيقَ وَبَيَانِ تَفْضِيلِ صَوْمِ يَوْمٍ وَإِفْطَارِ يَوْمٍ
This hadith has been narrated by Yahya b. Abu Kathir with the same chain of transmitters and he made this addition after these words:
During every month, (fasting) for three days, there is for you ten times for every good and that is perpetual fasting (for three days would bring a reward for full thirty days). I said: What is the fast of the Messenger of Allah, David? He said: Half of the age (observing fast on alternate days for the whole life). And in the hadith no mention has been made of the recital of the Qur'an, and he did not say: Your visitor has a right upon you, but (instead) he said: Your son has a right upon you.
Reference: Sahih Muslim 1159 c
In-book reference: Book 13, Hadith 237
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2588