Sayyidah Aishah (may Allah be pleased with her) said: Before the fasting of Ramadan was made obligatory, people used to fast on Ashura, and it was on that day that the Ka'bah was covered with its covering. When Allah made Ramadan obligatory, the Prophet (peace and blessings be upon him) said: "Whoever wishes to fast Ashura may fast it, and whoever wishes to leave it may leave it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9732