Sahih Muslim — The Book of Fasting — Hadith No: 2714
Translation
A'isha, the Mother of the Believers (Allah be pleased with her), reported that one day the Messenger of Allah may peace be upon him) said to me:'A'isha, have you anything (to eat)? I said: 'Messenger of Allah, there is nothing with us. Thereupon he said: I am observing fast. She said: The Messenger of Allah (ﷺ) went out, and there was a present, for us and (at the same time) some visitors dropped in. When the Messenger of Allah (ﷺ) came back, I said to him: Messenger of Allah, a present was given to us, (and in the meanwhile) there came to us visitors (a major Portion of it has been spent on them), but I have saved something for you. He said: What is it? I said: It is hais (a compound of dates and clarified butter). He said: Bring that. So I brought it to him and he ate it and then said: I woke up in the morning observing fast. Talha said: I narrated this hadith to Mujahid and he said: This (observing of voluntary fast) is like a person who sets apart Sadaqa out of his wealth. He may spend it if he likes, or he may retain it if he so likes