Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Volume 10, Page 401, Fatwa No.13700
Question: Is it permissible to fast ‘Aashooraa just one day?
Response: It is permissible to fast the day of ‘Aashooraa (tenth day of Muharram) just one day, however, it is better to fast the day before it or the day after it [also] and this is the established Sunnah of the Prophet ﷺ who said:
«If I am [still] here next year, then certainly I shall fast the ninth [day of Muharram]», [transmitted by Muslim, Ahmad, Ibn Maajah, Ibn Abee Shaybah, at-Tahaawee, al-Bayhaqee and al-Baghawee]. Ibn ‘Abbaass (radhi-yAllaahu ‘anhumaa) said: «along with the tenth [day of Muharram]».
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.
