Talhah bin Yahya (may Allah have mercy on him) narrated a hadith (from Aishah) that one day the Messenger of Allah (peace and blessings be upon him) came to me and he (peace and blessings be upon him) said: "Do you have anything?" We said: No. So he (peace and blessings be upon him) said: "Then I am fasting."
Ya'la bin Ubayd narrates likewise from Talhah that he (peace and blessings be upon him) said: "In that case I am fasting."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7915