It is permissible, but it is disliked as a matter of preference to fast Ashura alone without the ninth or the eleventh. If one fasts it alone, they have acted contrary to what is preferred, but they do not incur sin, as mentioned in Al-Bahr Al-Ra'iq 2: 287, and Hashiyat Al-Tahbih 1: 332; because it was narrated from Ibn Abbas, may Allah be pleased with him, who said: "When the Messenger of Allah, peace be upon him, fasted on the day of Ashura and commanded its fasting, they said: O Messenger of Allah, this is a day that the Jews and Christians venerate. The Messenger of Allah, peace be upon him, said: If Allah wills, we will fast on the ninth next year. But the next year did not come until the Messenger of Allah, peace be upon him, passed away." This is in Sahih Muslim 2: 797, and Sunan Abu Dawood 2: 327.