“Are you fasting tomorrow?”
“But it’s for 2 days…”
“The Jews fast one day. We fast two…”
According to Sunni Muslim tradition, Ibn Abbas narrates that Muhammad (Peace be upon him) came to Madina and saw the Jews fasting on the tenth day of Muharram. He asked, “What is this?” They said, “This is a good day, this is the day when Allah saved the Children of Israel from their enemy and Musa (Moses) fasted on this day.”
So Muhammad (Peace be upon him) recommended to be different from the Jews and recommended fasting two days instead of one. 9th and 10th or the 10th and 11th day of Muharram.
Reference: Day of Ashura