Reference: Fataawa Ramadhaan – Volume 2, Page 694, Fatwa No.699
al-Fataawa libni Baaz – Kitaab ad-Da’wah, Volume 2, Page 172
Question: I began fasting the six [days] of Shawwaal, however I was unable to complete them because of extenuating circumstances and work. Now I have two days remaining, so what do I do O Eminence (Shaykh), do I make these days up or is there any sin upon me?
Response: Fasting the six [days] of Shawwaal is a recommended act of worship and not an obligation. So for you is the reward of that which you fasted of them, and it is hoped for you the complete reward if that which prevented you from fasting them all was an Islaamically acceptable reason. The Prophet ﷺ said:
And there is nothing for you to make up for that which you left of them.
And with Allaah lies all success.