It was narrated from Shuaib bin 'Abdullah bin 'Amr that his father said:
"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.'"
Reference: Sunan an-Nasa'i 2396 In-book : Book 22, Hadith 307 English translation : Vol. 3, Book 22, Hadith 2398
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