Musnad Ahmad 32:681

Abu Kamil—whose name is Muzaffar ibn Mudrik—narrated to us, Hammad ibn Salamah narrated to us, from Thabit, from Anas ibn Malik: "The Prophet, may Allah bless him and grant him peace, would fast until it was said: 'He has fasted, he has fasted,' and he would break his fast until it was said: 'He has broken his fast, he has broken his fast.'"