Hadith 4:422
Ibn Mahdi narrated to us, from Hammad ibn Salamah, from Hisham, from his father, who said: The likeness of one who performs voluntary fasting while he has [outstanding] makeup days from Ramadan is like the likeness of one who performs voluntary prayer while he fears that the obligatory prayer might pass him by.
حدثنا بن مهدي عن حماد بن سلمة عن هشام عن أبيه قال مثل الذي يتطوع وعليه قضاء من رمضان مثل الذي يسبح وهو يخاف أن تفوته المكتوبة