It is narrated from Aisha (may Allah be pleased with her) that she said: The Messenger of Allah (peace and blessings be upon him) used to fast on the 10th of Muharram and would also command (others) to fast on this day.
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 628
Hadith Takhrijصحیح: اخرجه ابن ماجة، الصيام، باب صيام يوم عاشوراء (1733) ۔ والدارمي (1767) ۔ والبيهقي في المعرفة السنن والآثار (2585)۔