There are enemies of Islam who also bring these hadiths Imam Muhammad Baqir said fasting in ashura eliminates the sin of an entire year (tahdihib al ahkam graded sahih by al khoei)

Imam Musa Kazim said Messenger of Allah would fast on the day of ashura (tahdihib al ahkam graded reliable by al majlisi)

How do we reply to them

Yes these narrations do exist. However position of many of our ulema today after studying the two sets of Hadith is that it’s not recommended to fast and should be avoided