سنن النسائي

It was narrated that 'Amr bin Shurahbil said: "A man came to the Messenger of Allah (ﷺ) 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 (ﷺ) 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 (ﷺ) said: 'Whoever fasts every day of his life, then he has not fasted."