Musnad Abu Ya'la al-Mawsili 150:239

Abu Sa'id al-Ashajj narrated to us, Abu Khalid narrated to us, from Layth, from Mujahid, from 'Aishah, she said: The Messenger of Allah (may Allah bless him and grant him peace) would seek lunch, and we would say: "There is none." He would say: "Indeed, I am fasting."