"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
← Previous Hadith '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 Next Hadith → "The Messenger of Allah said: 'I have heard that you stand (in prayer) all night and fast all day.' I said: 'O Messenger of Allah, I only meant good by that.' He said: 'One who fasts every day of his life has not truly fasted. But I will tell you what fasting for a lifetime means: Three days of every month.' I said: 'O Messenger of Allah, I said: 'I am able to do more than that.' He said 'Observe the fast of Dawud, peace be upon him; he used to fast one day and not the next."' '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 308