Abu Dawud Book 2, Hadith Number 1390.
Chapter : On fixing a part from the qran for daily recitation.
Narated By Abdullah ibn Amr ibn al-‘As : Wahb ibn Munabbih said: Abdullah ibn Amr asked the Prophet (PBUH); In how many days should one complete the recitation of the Qur’an? He said: In forty days. He then said: In one month. He again said: In twenty days. He then said: In fifteen days. He then said: In ten days. Finally he said: In seven days.