Hadith 4:881
9747. Al-Thaqafi narrated to us, from Humayd, from Anas, that the Messenger of Allah (may Allah bless him and grant him peace) used to fast from the month until we would say, "He does not break his fast," and he would break his fast until we would say, "He does not fast any part of it."
حدثنا الثقفي عن حميد عن أنس أن رسول الله صلى الله عليه وسلم كان يصوم من الشهر حتى نقول ما يفطر ويفطر حتى نقول ما يصوم منه شيئا