Question: If someone misses ten days of fasting in Ramadaan due to a Shar‘ee (Islaamic legal) excuse, and then fasts the six days of Shawwaal after Ramadaan without making up for the missed days of Ramadaan, will they attain the Thawaab (reward from Allaah) promised for fasting the whole month of Ramadaan then following it with fasting the six days of Shawwaal? Will it be as if they have fasted for the whole year? Guide us, and may Allaah reward you with the best.
Answer: Determining the rewards of the deeds performed by a servant of Allaah is the special domain of Allaah (Glorified and Exalted be He). If a servant seeks the reward of Allaah (Glorified and Exalted be He), and exerts themselves in their obedience to Allaah, their reward will not be lost. Allaah (Exalted be He) says: …certainly We shall not make the reward of anyone who does his (righteous) deeds in the most perfect manner to be lost. Anyone who still has missed days of Ramadaan which they need to make up for should first fast them, and then fast the six days of Shawwaal. Following the Sawm (Fast) of Ramadaan with six days of Shawwal will not be fulfilled unless one has first completed the Sawm of Ramadaan... read more here.