Sahih al-Bukhari 1978
Sahih al-Bukhari · Book 30: Fasting · Saum (fasting) on alternate days
Narrated Mujahid from `Abdullah bin `Amr:
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا غُنْدَرٌ ، حَدَّثَنَا شُعْبَةُ ، عَنْ مُغِيرَةَ ، قَالَ سَمِعْتُ مُجَاهِدًا ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو ـ رضى الله عنهما ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " صُمْ مِنَ الشَّهْرِ ثَلاَثَةَ أَيَّامٍ ". قَالَ أُطِيقُ أَكْثَرَ مِنْ ذَلِكَ. فَمَا زَالَ حَتَّى قَالَ " صُمْ يَوْمًا وَأَفْطِرْ يَوْمًا " فَقَالَ " اقْرَإِ الْقُرْآنَ فِي شَهْرٍ ". قَالَ إِنِّي أُطِيقُ أَكْثَرَ. فَمَا زَالَ حَتَّى قَالَ فِي ثَلاَثٍ.
The Prophet (ﷺ) said (to `Abdullah), "Fast three days a month." `Abdullah said, (to the Prophet) "I am able to fast more than that." They kept on arguing on this matter till the Prophet (ﷺ) said, "Fast on alternate days, and recite the whole Qur'an once a month." `Abdullah said, "I can recite more (in a month)," and the argument went on till the Prophet (ﷺ) said, "Recite the Qur'an once each three days." (i.e. you must not recite the whole Qur'an in less than three days).