Musnad Ahmad 28:299

Bahz narrated to us, Hammam narrated to us, from Qatadah, from Yazid, the brother of Mutarrif, from Abdullah ibn Amr, that he asked the Prophet, peace and blessings of Allah be upon him: "In how long should I recite the Quran?" — then he mentioned the hadith — he said, until he said: "In seven [days]; he does not understand it, the one who recites it in less than three." And he said: "How should I fast?" He said: "Fast three days of every month, one day out of every ten days, and the reward of nine days will be written for you." He said: "I am stronger than that." He said: "Fast two days out of every ten days, and the reward of eight days will be written for you," until he reached five days.