Musnad Ahmad 28:288

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Husayn, from Mujahid, from ‘Abdullah ibn ‘Amr: That he married a woman from Quraysh, and he would not approach her, as fasting and prayer would occupy him. This was mentioned to the Prophet, peace and blessings of Allah be upon him, so he said: "Fast three days of every month." He said: "I am capable of more than that." He continued with him until he said to him: "Fast one day and break your fast one day." And he said to him: "Recite the Quran every month." He said: "I am capable of more than that." He said: "Recite it every fifteen [days]." He said: "I am capable of more than that." He said: "Recite it every seven [days]," until he said: "Recite it every three [days]." And the Prophet, peace and blessings of Allah be upon him, said: "Indeed, for every action there is an intensity, and for every intensity there is a slackening. So whoever’s slackening is toward my Sunnah, then he has succeeded, and whoever’s slackening is toward other than that, then he has perished."