Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 10, Page 401, Fatwa No.6774
Question: What is the ruling regarding the one who has days of fasting remaining from the month of Ramadhaan yet wishes to fast voluntary fasts or fast the day of ‘Aashooraa (tenth day of Muharram)?
For example, he wants to fast both the tenth and eleventh days with the intention of making up [the missed fasts of Ramadhaan] and not the fast of the day of ‘Aashooraa. Also, is it permissible for one to fast the day of ‘Aashooraa, even if he has fasts to make up from the month of Ramadhaan?
And is it permissible for one who has days to make up from Ramadhaan to fast the Day of ‘Aashooraa and the day before it or the day after it, with the intention of making up (what he has missed)?
Response: He should not do the voluntary fast whilst he has a day or days to make up from Ramadhaan. Rather, he should begin by making up that which he has [outstanding] from Ramadhaan, then do the voluntary fast. Secondly, if he fasted the tenth and eleventh days of the month of Muharram with the intention of making up that which he has [outstanding] from the days he did not fast in the month of Ramadhaan, then that is permissible and sufficient in making up two days from that which he has outstanding; For that which the Prophet (sal-Allaahu ‘alayhi wa sallam) said:
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.