Abu Dawud Book 2, Hadith Number 1384.

Narated By ‘Abd Allah b. ‘Amr : The Apostle of Allah (PBUH) said to me: Keep fast for three days every month, and finish the recitation of the Qur’an in one month. I and he differed among ourselves over the period of time. He said: Fast one day and give it up the other day. The narrator of ‘Ata said: The people differed from my father (in narrating the period of time). Some narrated seven days and other five.