Sahih Muslim Book 6, Hadith Number 2574.

Chapter : Permissibility of making an intention for voluntary fasting before noon and permissibility of breaking voluntary fast without reason.

‘Aisha, the Mother of the Believers (Allah be pleased with her), reported: The Apostle of Allah (may peace be upon him) came to me one day and said: Is there anything with you (to eat)? I said: No. Thereupon he said: I shall then be fasting. Then he came to us another day and we said: Messenger of Allah, hais has been offered to us as a gift. Thereupon he said: Show that to me; I had been fasting since morning. He then ate it.