Umm al-Mu'minin Sayyidah Aishah (may Allah be pleased with her) narrates: One day the Messenger of Allah (peace and blessings be upon him) said to me: "O Aishah, do you have anything?" I said: No, by Allah, we have nothing. He (peace and blessings be upon him) said: "Then I am fasting." She said: The Messenger of Allah (peace and blessings be upon him) went out, and a gift was given to us, or some visitors came to us. When the Messenger of Allah (peace and blessings be upon him) returned, I said: O Messenger of Allah, a gift was given to us, or some visitors came to us, and I have kept something aside for you. He (peace and blessings be upon him) said: "What is it?" I said: It is hays. He (peace and blessings be upon him) said: "Bring it to me." I brought it and he (peace and blessings be upon him) ate. Then he (peace and blessings be upon him) said: "I had in fact begun this morning with the intention of fasting."
Imam Muslim narrated it with an addition: Talhah (may Allah have mercy on him) said: I narrated this hadith to Mujahid (may Allah have mercy on him), and he said: That is like a man who sets aside charity from his wealth; if he wishes he gives it, and if he wishes he withholds it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8339