Musnad Abu Ya'la al-Mawsili 117:261

Zuhayr narrated to us, Rawh ibn ‘Ubadah narrated to us, Hammad informed us, from Thabit, from Anas, that the Prophet (peace be upon him) used to fast until it was said, "He has fasted," and he would break his fast until it was said, "He has broken his fast."