Abu Dawud Book 2, Hadith Number 1383.

Narated By ‘Abd Allah b. ‘Amr : The Prophet (PBUH) said to me: Complete the recitation of the Qur’an in one month. He said: I have more strength. He (the Prophet) said: Complete the recitation in twenty days. He again said: I have more energy. He said: Recite in fifteen days. He again said: I have more energy. He said: Recite in ten days. He again sais: I have more energy. He said: Recite in seven days; do not add to it.

Abu Dawud said: The tradition narrated by Muslim is more perfect.