سنن نسائی · Hadith 2394

It was narrated that ‘Amr bm Shurahbil said. “A man came the Messenger of Allah صلی اللہ علیہ وآلہ وسلم and said: ‘Messenger of Allah صلی اللہ علیہ وآلہ وسلم what do you say about a man who’fasted for the rest of his life?’ The Messenger of Allah صلی اللہ علیہ وآلہ وسلم 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: ‘That is too much.’ 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.” (Sahih).