Sayyidah Aishah (may Allah be pleased with her) narrates that the Messenger of Allah (peace and blessings be upon him) "used to order the fasting of the day of Ashura" before Ramadan was made obligatory. When the fasting of Ramadan was made obligatory, whoever wished would fast, and whoever wished would not fast.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8410