Musnad Ahmad 28:290

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Habib, from Abu al-‘Abbas, from ‘Abdullah ibn ‘Amr, that the Messenger of Allah (ﷺ). And Rawh narrated to us, Shu‘bah narrated to us, I heard Habib ibn Abi Thabit, I heard Abu al-‘Abbas the poet—and he was truthful—narrating from ‘Abdullah ibn ‘Amr, who said: The Messenger of Allah (ﷺ) said: "O ‘Abdullah ibn ‘Amr, you fast the entire time. If you fast the entire time and stand [in prayer] the night, the eye will sink in, and the soul will become exhausted. He has not fasted who fasts forever. Fast three days of the month; [that is] the fasting of the entire time." I said: "I am capable of more." He said: "Fast the fasting of Dawud (David), for he used to fast one day and break his fast one day, and he would not flee when he met [the enemy]." And Rawh said: "The soul will become exhausted (nahathat)."