Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) mentioned Ashura and said: "It was the fast of the people of the Jahiliyyah, so whoever wishes may fast it and whoever wishes may leave it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13126