If somebody asks you, “since when is fasting on the 10th MuHarram Sunnah?”

al-Nawawiyy said: “The scholars agreed that fasting on the day of Ashooraa’ is recommended (Sunnah) and not obligatory. They differed regarding its ruling at the beginning of this Islamic nation when fasting on this day was first prescribed before the fasting of RamaDaan. Abu Haneefah said: It was obligatory. The companions of ash-Shafi^iyy differed on this matter with two well-known views. The more well-known view among them is that it was always recommended since it was prescribed and never obligatory for this nation, but it was highly recommended. When the fasting of RamaDaan was prescribed, it became recommended but not to the same extent. The second view is that it has been obligatory, as Abu Haneefah said.”