Musnad Ahmad 28:401

'Abd al-Wahhab ibn 'Ata narrated to us, al-Jariri informed me, from Abu al-'Ala', from Mutarrif ibn 'Abd Allah, from 'Abd Allah ibn 'Amr, who said: I came to the Messenger of Allah (peace and blessings of Allah be upon him) and said: "O Messenger of Allah, command me to fast." He said: "Fast one day, and you shall have the reward of nine." I said: "O Messenger of Allah, I find strength, so increase me." He said: "Fast two days, and you shall have the reward of eight days." I said: "O Messenger of Allah, I find strength, so increase me." He said: "Fast three days, and you shall have the reward of seven days." He continued to decrease for me until he said: "Indeed, the best of fasting is the fasting of my brother Dawud, or the Prophet of Allah Dawud—al-Jariri was in doubt—fast one day and break your fast one day." 'Abd Allah said, when he became weak: "I wish I had been content with what the Prophet (peace and blessings of Allah be upon him) had commanded me."