Narrated Abdullah ibn Amr ibn al-‘As: Yazid ibn Abdullah (RA) said that Abdullah ibn Amr (RA) asked the Prophet (peace be upon him): In how many days should I complete the recitation of the whole Qur’an, Messenger of Allah? He replied: In one month. He said: I am more energetic to complete it in a period less than this. He kept on repeating these words and lessening the period until he said: Complete its recitation in seven days. He again said: I am more energetic to complete it in a period less than this. The Prophet (peace be upon him) said: He who finishes the recitation of the Qur’an in less than three days does not understand it.
Sunan Abi Dawud, Book 6, Hadith 20