Reference: Siyar A’laam an-Nubalaa – Volume 5, Page 326
Abu Jabalah said:
I was travelling with Ibn Shihaab az-Zuhree, and he fasted the day of ‘Aashooraa. It was then said to him: Why do you fast [today], yet do not fast when you are also travelling in Ramadhaan?
He replied: Ramadhaan has plenty of other days [throughout the year in which the missed fasts can be made up], but [the missed fast of] ‘Aashooraa cannot be made up.