Sahih Bukhari, Chapter: Fasting

from Abdullah bin Amr, The Prophet 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 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 said, "Recite the Quraan once each three days." (i.e. you must not recite the whole Quraan in less than three days).