Musnad Ahmad 28:402

'Abd al-Wahhab ibn 'Ata narrated to us, Muhammad ibn 'Amr informed me, from Abu Salamah, from 'Abdullah ibn 'Amr, that the Messenger of Allah (ﷺ) entered upon him in his house and said: "O 'Abdullah ibn 'Amr, have I not been informed that you burden yourself with standing in prayer at night and fasting during the day?" He said: "Indeed, I do that." He said: "It is sufficient for you—and I do not say [you must] do it—that you fast three days of every month; for the good deed is ten times its like, so it is as if you have fasted the entire year." He said: "So I made it difficult, and he made it difficult for me." He said: "I said: 'I find strength for more than that.'" He said: "It is sufficient for you that you fast three days of every week." He said: "So I made it difficult, and he made it difficult for me." I said: "I find strength in myself." The Prophet (ﷺ) said: "The most just fasting in the sight of Allah is the fasting of Dawud (David): half of the year." Then he said: "Your self has a right over you, and your family has a right over you." 'Abdullah used to fast that fast, until when old age and weakness overtook him, he would say: "That I had accepted the concession of the Messenger of Allah (ﷺ) is more beloved to me than my family and my wealth."