Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.3089
Fataawa Ramadhaan – Volume 2, Page 749, Fatwa No.760
Question: Is it possible for a person to fast three days and nights in Ramadhaan, such that this becomes a substitute for [the] thirty days [of Ramadhaan]?
Response: This is not permissible and none of the people of knowledge (scholars) have said this. This is because the night is not the time for fasting, and he who does so is regarded as one who opposes the pure Sharee’ah by introducing something which Allaah has not legislated and as such ends up not fasting in Ramadhaan without [a valid Islaamic] reason. Allaah (subhaanahu wa ta’aala) has legislated upon able and qualified Muslims to fast all of Ramadhaan, and it is not sufficient to fast part of it in place of the rest.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.