Sayyiduna Ammar bin Yasir (may Allah be pleased with him) says: Before the command of Ramadan was revealed, we were commanded (to fast) Ashura. When the command of Ramadan was revealed, we were no longer commanded to do so.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5130