(78) باب صَوْمِ عَشْرَةِ أَيَّامٍ مِنَ الشَّهْرِ وَاخْتِلاَفِ أَلْفَاظِ النَّاقِلِينَ لِخَبَرِ عَبْدِ اللَّهِ بْنِ عَمْرٍو فِيهِ
It was narrated that 'Abdullah bin 'Amr said:
"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."
Reference: Sunan an-Nasa'i 2397
In-book reference: Book 22, Hadith 308
English translation: Vol. 3, Book 22, Hadith 2399