Sahih Al Bukhari Hadith Number 1978

Narrated Mujahid from `Abdullah bin `Amr: The Prophet (peace be upon him) 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 (peace be upon him) 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 (peace be upon him) said, “Recite the Qur’an once each three days.” (i.e. you must not recite the whole Qur’an in less than three days).