'Amr said: "I spoke to the Prophet and he said: 'Fast one day out of ten and you will have the reward of the other nine.' I said: 'I am able to do more than that.' He said: 'Fast one day out of eight and you will have the reward of the other eight.' I said: 'I am able to do more than that.' He said: 'Fast one day out of eight and you will have the reward of the other seven.' I said: 'I am able to do more than that.' 'Fast one day and not the next."'
Sunan an-Nasa'i Book 22, Hadith 306
← Previous Hadith "Fast one day, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast two days, and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast three days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Fast four days and you will have the reward of what is left." He said: "I am able to do more than that." He (ﷺ) said: "Observe the best kind of fasting before Allah, the fast of Dawud, peace be upon him; he used to fast one day and break his fast for one day." 'Ata said: "Someone who heard him told me that Ibn 'Umar (said) that the Prophet said: 'Whoever fasts every day of his life, then he has not fasted.'"

Sunan an-Nasa'i Book 22, Hadith 305 Next Hadith → "The Messenger of Allah said to me: 'Fast one day and you will have the reward of ten.' I said: 'Let me fast more.' He said: 'Fast two days and you will have the reward of nine.' I said: 'Let me fast more than that.' He said: 'Fast three days and you will have the reward of eight." (One of the narrators) Thabit said: "I mentioned that to Mutarrif and he said: 'I only see that he is making more effort for less reward.'"

Sunan an-Nasa'i Book 22, Hadith 307