Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 199
Narrated Mujahid 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 Qur'an once each three days." (i.e. you must not recite the whole Qur'an in less than three days).