Musnad Ahmad 32:1706

'Affan narrated to us, Hammad narrated to us, he said: Thabit informed me, from Anas: "That the Messenger of Allah, peace and blessings of Allah be upon him, used to fast until it would be said he has fasted, and he would break his fast until it would be said he has broken his fast." And he said once: "He has broken his fast, he has broken his fast, he has broken his fast."