It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) was mentioned about the fasting of the day of Ashura. The Messenger of Allah (peace and blessings be upon him) said: "The people of ignorance also used to fast on this day. Whoever wishes may fast on this day, and whoever dislikes may not fast."
Hadith Referenceالسنن المأثورة / باب ما جاء في صيام عاشوراء / 324
Hadith Takhrijصحيح مسلم، الصيام، باب صوم يوم عاشوراء، رقم : 1126