Ahlul Bayt (AS) mentioned that fasting the day of Ashura is the fasting of Ibn Marjanah, the ruler of Yazeed on Iraq ذلك صوم ابن مرجانة

Yazeed and his followers wanted to mislead Muslims and make them celebrate the day of Ashura to cover their crime in killing Imam Husain on that day. Many Sunni scholars including leading Wahabi Shaikh Naasir Al-Deen Al-Albani said that narrations on fasting in Ashura were fabricated by the enemies of Ahlul Bayt (Nawaasib)(Tamaam al-Minnah, page 412). Many Sunni scholars including Al-Albani said that fasting in the day of Ashura is a Bid'ah.

Also 'Ibn Al-Jawzi in Al-Mawdoo'aat ,

and  Ibn Al-Soyooti in Al-la'aali Al-Masnoo'ah Fi Al Al-Ahaadeeth Al-Mawdhoo'ah.

and Ibn Iraq in Tabzeeh Al-Sharee'ah

and Al-Manaawi from Al-Majid Al-loghawi,

and Mohammad Salim Al-Munajjid .

The prominent Sunni scholar al-Maqreezi that Al-Hajjaj during the rule of Abdulmalik ibn Marwah ordered to announce the day of Ashura as Eid then it was taken by Ayyoubi kings who spread it among Sunni masses.

'According to Al-Kaafi, Volume 4, page 146, from Ja'far ibn Easa said: I asked (Imam) Al-Radha (AS) about fasting on the day of Ashura and what people say about it, he said: Do you ask me about the fasting of Ibn Marjaanah? That is day taken as a day of fasting by the murderers of imam Hussain(As).
'Wassalam.