(58) باب صَوْمِ يَوْمٍ وَإِفْطَارِ يَوْمٍ
Narrated Mujahid from `Abdullah bin `Amr:
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).
Reference: Sahih al-Bukhari 1978
In-book reference: Book 30, Hadith 85
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 199