Muhammad Ibn Hasan At-Tusi writes:

فالوجه في الجمع بين هذه الاخبار ما كان يقول شيخنا رحمه الله وهو أن من صام يوم عاشوراء على طريق الحزن بمصاب آل محمد صلى الله عليهم والجزع لما حل بعترته فقد أصاب، ومن صامه على ما يعتقد فيه مخالفونا من الفضل في صومه والتبرك به والاعتقاد لبركته وسعادته فقد أثم وأخطأ

“The traditions can be reconciled with the point of view that our teacher (Al-Mufid), May God rest his soul, addressed, namely, that the person who, on the day of Ashura’, was affected by the misfortune of the family of Muhammad (s) and who fasted out of sadness over what happened to his relatives, was correct and the person who fasted on that day because of the conviction of our opponents that it was considered to be an excellence of that day and an attainment of blessings and out of the belief in its bestowal and bliss, he sinned and was wrong.” [Al-Istibsar, volume 2 page 135 – 136]