Reference: Fataawa Ramadhaan – Volume 2, Page 702, Fatwa No.708
Majmoo’ Fataawa Ibn Baaz – Volume 3, Page 269
Question: I fasted two days of al-ayyaam al-beedh (13th, 14th and 15th of the lunar month) and was unable to fast the third day due to extenuating circumstances, so will I receive the reward for the two days [I did fast]?
Response: No doubt you will receive the reward for them (the two days fasted), so long as you fasted them purely for the Sake of Allaah without showing off and without seeking recognition; Allaah (‘Azza wa Jall) says:
And that which has been mentioned with a similar meaning from [other] aayaat and ahaadeeth.
And Allaah is the Expounder of All Success.