(75) باب صَوْمِ ثُلُثَىِ الدَّهْرِ وَذِكْرِ اخْتِلاَفِ النَّاقِلِينَ لِلْخَبَرِ فِي ذَلِكَ
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."
Reference: Sunan an-Nasa'i 2385
In-book reference: Book 22, Hadith 296
English translation: Vol. 3, Book 22, Hadith 2387