Musnad Ahmad 28:1

Hashim narrated to us, from Husayn ibn Abd al-Rahman and Mughirah al-Dabbi, from Mujahid, from Abdullah ibn Amr, who said: My father married me to a woman from Quraysh. When she entered upon me, I did not approach her due to the strength I possessed for worship, in terms of fasting and prayer. Amr ibn al-Aas came to his daughter-in-law and entered upon her. He said to her: "How did you find your husband?" She said: "He is the best of men, or like the best of husbands; he has not inspected our private quarters, nor has he known our bed." He came to me, rebuked me, and bit me with his tongue, saying: "I married you to a woman of Quraysh of noble lineage, yet you have withheld her rights and done such and such." Then he went to the Prophet, peace and blessings of Allah be upon him, and complained about me. The Prophet, peace and blessings of Allah be upon him, sent for me, and I came to him. He said to me: "Do you fast during the day?" I said: "Yes." He said: "And do you pray during the night?" I said: "Yes." He said: "But I fast and I break my fast, I pray and I sleep, and I touch women. So whoever turns away from my Sunnah is not of me." He said: "Recite the Quran every month." I said: "I find myself stronger than that." He said: "Then recite it every ten days." I said: "I find myself stronger than that." One of them—either Husayn or Mughirah—said: He said: "Then recite it every three days." He said: Then he said: "Fast three days every month." I said: "I am stronger than that." He continued to increase for me until he said: "Fast one day and break your fast one day, for that is the best of fasting, and it is the fasting of my brother Dawud, peace and blessings of Allah be upon him." Husayn said in his narration: Then the Prophet, peace and blessings of Allah be upon him, said: "For every worshiper there is an intensity, and for every intensity there is a slackening. So it is either towards a Sunnah or towards an innovation. Whoever’s slackening is towards a Sunnah has been guided, and whoever’s slackening is towards other than that has perished." Mujahid said: When Abdullah ibn Amr became weak and old, he would fast those days in that manner, joining some of them to others to gain strength, then he would break his fast after those days. He said: And he would recite his portion in that manner, sometimes increasing and sometimes decreasing, but he would fulfill the number, either in seven days or in three. He said: Then he would say after that: "That I had accepted the concession of the Messenger of Allah, peace and blessings of Allah be upon him, would be more beloved to me than anything equivalent to it, but I parted from him upon a matter that I dislike to oppose for something else."