Musannaf Ibn Abi Shaybah

Ibrahim considered it disliked for a man to observe voluntary fasts while he still has missed fasts of Ramadan to make up.

Hasan considered it disliked for a man to observe voluntary fasts while he still has missed fasts of Ramadan to make up, however, he may fast the ten days of Dhul-Hijjah.

Hazrat Urwah says that the example of a person who keeps voluntary fasts while the missed fasts of Ramadan are still due is like the person who is engaged in offering voluntary prayers while there is a fear of missing the obligatory prayer.

Hazrat Malik bin Anas (may Allah be pleased with him) narrates that Hazrat Sulaiman bin Yasar and Hazrat Saeed bin Musayyib were asked about a person who keeps a voluntary fast while he still has missed fasts of Ramadan to make up; both of them considered it disliked.