Musnad Ahmad 28:398

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us; and Rawh said: Ibn Jurayj narrated to us, he said: I heard 'Ata' claiming that Abu al-'Abbas, the poet, informed him that he heard 'Abdullah ibn 'Amr saying: It reached the Prophet, may Allah bless him and grant him peace, that I fast continuously and pray throughout the night. He said: Either he sent for me or I met him, and he said: "Have I not been informed that you fast and do not break it, and that you pray throughout the night? Do not do that, for your eye has a share, your self has a share, and your family has a share. So fast and break your fast, pray and sleep, and fast one day out of every ten days, and you will have the reward of nine." I said: "I find myself stronger than that, O Prophet of Allah." He said: "Then fast the fast of David." I said: "And how did David fast, O Prophet of Allah?" He said: "He used to fast one day and break his fast one day, and he would not flee when he met [the enemy]." I said: "Who can attain this for me, O Prophet of Allah?" — 'Ata' said: I do not know how he mentioned the fasting of eternity —. The Prophet, may Allah bless him and grant him peace, said: "He has not fasted who fasts forever." 'Abd al-Razzaq and Rawh said: "He has not fasted who fasts forever," twice.