It is narrated from Sayyiduna 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 / 16132
Hadith Gradingحکم دارالسلام:إسناده ضعيف جدا، لضعف ثوير بن أبى فاخته