Musnad Ahmad 28:608

'Arim narrated to us, Mu'tamir narrated to us, from his father, Abu al-'Ala' narrated to us, from Mutarrif, from Ibn Abi Rabi'ah, from 'Abdullah ibn 'Amr, who said: I mentioned fasting to the Prophet, peace and blessings of Allah be upon him, and he said: "Fast one day out of every ten days, and you will have the reward of the nine." I said: "I am stronger than that." He said: "Then fast one day out of every nine days, and you will have the reward of the eight." I said: "I am stronger than that." He said: "Then fast one day out of every eight days, and you will have the reward of those seven." I said: "I am stronger than that." He said: He did not cease until he said: "Fast one day and break your fast one day."