Musnad Ahmad 8:155

Muhammad ibn Bishr narrated to us, Isma'il ibn Abi Khalid narrated to us, from Muhammad ibn Sa'd, from his father Sa'd, who said: The Messenger of Allah (may Allah bless him and grant him peace) came out to us while he was striking one of his hands against the other, and he was saying: "The month is like this and like this." Then he withheld one of his fingers on the third [time].