Sunan an-Nasa'i 2385
It was narrated from 'Amr bin Shurahbil that a man from among the Companions of the Prophet said:
'It was said to the Prophet that a man fasted for the rest of his life. He said: 'I wish that he had never eaten.' They said: 'Two-thirds (of a lifetime)?' He said: 'That is too much.' Then he said: 'Shall I not tell you of that which will take away impurity from the heart? Fasting three days each month." '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."
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّحْمَنِ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الأَعْمَشِ، عَنْ أَبِي عَمَّارٍ، عَنْ عَمْرِو بْنِ شُرَحْبِيلَ، عَنْ رَجُلٍ، مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم قَالَ قِيلَ لِلنَّبِيِّ صلى الله عليه وسلم رَجُلٌ يَصُومُ الدَّهْرَ ‏.‏ قَالَ ‏"‏ وَدِدْتُ أَنَّهُ لَمْ يَطْعَمِ الدَّهْرَ ‏"‏ ‏.‏ قَالُوا فَثُلُثَيْهِ قَالَ ‏"‏ أَكْثَرَ ‏"‏ ‏.‏ قَالُوا فَنِصْفَهُ قَالَ ‏"‏ أَكْثَرَ ‏"‏ ‏.‏ ثُمَّ قَالَ ‏"‏ أَلاَ أُخْبِرُكُمْ بِمَا يُذْهِبُ وَحَرَ الصَّدْرِ صَوْمُ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ ‏"‏ ‏.‏