It is narrated from Sayyiduna Ibn Zubair that today is the day of Ashura, so observe its fast because the Prophet (peace and blessings be upon him) instructed to fast on this day.
Hadith Referenceمسند احمد / مسند المدنيين رضي الله عنهم اجمعين / 16119
Hadith Gradingحکم دارالسلام:إسناده ضعيف جدا لضعف ثوير بن أبى فاختة