Musnad Ahmad 28:387

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mughirah, I heard Mujahid narrating from Abdullah ibn Amr, from the Prophet, peace and blessings of Allah be upon him, that he said: "Fast three days of the month." He said: "I am able to do more than that." He said: "He did not cease until he said: 'Fast one day and break your fast one day.'" Then he said to him: "Recite the Quran in every month." He said: "I am able to do more than that." He said: "He did not cease until he said: 'Recite the Quran in every three [days].'"