Hadith 4:423
Ibn Mahdi narrated to us, from Malik ibn Anas, who said: Sulayman ibn Yasar and Sa’id ibn al-Musayyib were asked about a man who performs a voluntary fast while he has [days of] making up [missed fasts] from Ramadan, and they disliked that.
حدثنا بن مهدي عن مالك بن أنس قال سئل سليمان بن يسار وسعيد بن المسيب عن رجل تطوع وعليه قضاء من رمضان فكرها ذلك