The Opportunity of Fasting Ashura

It is reported that Imām Al-Zuhrī – Allāh have mercy on him – was traveling and fasted ʿĀshūrāʾ, so he was asked:

“How comes you fast the Day of ʿĀshūrāʾ when traveling but not so during the fast of Ramaḍān?” He replied, “For Ramaḍān, the number of days can be made up later, but ʿĀshūrāʾ will be missed.”

Al-Bayhaqī, Shuʿab Al-Īmān ḥadīth 3518.