Reference: Fataawa Ramadhaan – Volume 2, Page 707, Fatwa No.716
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.8966
Question: My habit is to fast Mondays, Thursdays and Fridays every week, so is there any harm in that?
Response: If the matter is as you have mentioned, then you have done well so long as you do not single out Fridays alone for fasting, because the Prophet ﷺ said:
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.