It was narrated that 'Aishah said:"The Messenger of Allah passed by my door. He said: 'Do you have anything (to eat)?' I said 'I do not have anything.' He said 'Then I am fasting.' Then he passed by my door a second time and we had been given some Hais. I brought it to him and he ate, and I was surprised. I said: 'O Messenger of Allah, you were fasting, then you ate Hais.' He said: 'Yes, O 'Aishah. The one who observes a fast other than in Ramadan, or making up a missed Ramadan, fast, is like a man who allocated some of is wealth to give in charity; if he wishes he may go ahead and give it, and if he wishes he may keep it

English translation · Sunan an-Nasa'i