Sunan an-Nasa'i 2386
It was narrated that 'Amr bin Shurahbil said:
"A man came to the Messenger of Allah and said: 'O Messenger of Allah, what do you say about a man who fated for the rest of his life?' The Messenger of Allah said: 'I wish that he would never eat for the rest of his life.' He said: 'Two-thirds ( of a lifetime)?' he said: 'That is too much.' He said: 'Half?' He said: 'Shall I not tell you of that which will take away impurity from the heart?' He said: 'Yes.' He said: 'Fasting for 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 الْعَلاَءِ، قَالَ حَدَّثَنَا أَبُو مُعَاوِيَةَ، قَالَ حَدَّثَنَا الأَعْمَشُ، عَنْ أَبِي عَمَّارٍ، عَنْ عَمْرِو بْنِ شُرَحْبِيلَ، قَالَ أَتَى رَسُولَ اللَّهِ صلى الله عليه وسلم رَجُلٌ فَقَالَ يَا رَسُولَ اللَّهِ مَا تَقُولُ فِي رَجُلٍ صَامَ الدَّهْرَ كُلَّهُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ وَدِدْتُ أَنَّهُ لَمْ يَطْعَمِ الدَّهْرَ شَيْئًا ‏"‏ ‏.‏ قَالَ فَثُلُثَيْهِ قَالَ ‏"‏ أَكْثَرَ ‏"‏ ‏.‏ قَالَ فَنِصْفَهُ قَالَ ‏"‏ أَكْثَرَ ‏"‏ ‏.‏ قَالَ ‏"‏ أَفَلاَ أُخْبِرُكُمْ بِمَا يُذْهِبُ وَحَرَ الصَّدْرِ ‏"‏ ‏.‏ قَالُوا بَلَى ‏.‏ قَالَ ‏"‏ صِيَامُ ثَلاَثَةِ أَيَّامٍ مِنْ كُلِّ شَهْرٍ ‏"‏ ‏.‏