Why Fast on ʿĀshūrāʾ?

Imām al-Zuhrī [d. 124 AH] (rahimahullah) was once travelling and it was said to him: Why are you fasting (ʿĀshūrāʾ - a voluntary fast) while you break your fasts during travels in Ramaḍān?

He responded: Ramaḍān has other days (which can be made up) while ʿĀshūrāʾ will expire.

[Translated by Munīb al-Ṣumālī | TROID]

السير للذهبي ٣٤٢/٥