"The Messenger of Allah came to me saying, 'Do you have anything for breakfast?' I said: 'No.' Then he said: 'I am fasting.'" She said: "One day he came to me and I said: 'O Messenger of Allah; I have received a gift for us.' He said: 'What is it?'" She said: "I said 'Hais' He said: 'I began the day fasting'" She said: "Then he ate."
Jami` at-Tirmidhi Book 8, Hadith 53