Ayyub bin Abdullah bin Yasar (may Allah have mercy on him) said: I heard Mu'awiyah (may Allah be pleased with him), in Makkah al-Mukarramah on the day of Ashura, saying: The Messenger of Allah (peace and blessings be upon him) fasted on it and said: "Whoever among you has not fasted, let him fast it."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16885