Shaikh Uthaimeen, may Allaah have mercy on him, said, “The Sunnah is that a person fasts the six days of Shawwaal after finishing the make-up fasts [qadaa] of Ramadaan not before. So if he had some fasts to make up [due to having missed some in Ramadaan] but he fasted the six days of Shawwaal before the make-up fasts, he will not obtain their reward [i.e., the reward of the six of Shawwaal], because the Prophet صلى الله عليه وسلم said, “Whoever fasts Ramadaan …” and it is not correct to say about a person who has some fasts from Ramadaan still incumbent upon him that he has fasted Ramadaan, rather he has only fasted some of it.”

Ash-Sharh al-Mumti, vol. 6, p. 466.