Musnad Ahmad 30:1500

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Aswad narrated to us, from Abdullah ibn Rafi', from Abu Hurairah, from the Messenger of Allah, may Allah bless him and grant him peace, who said: "Whoever reaches Ramadan while he has something from Ramadan [that he has] not made up, it will not be accepted from him. And whoever fasts a voluntary fast while he has something from Ramadan [that he has] not made up, it will not be accepted from him until he fasts it."