Sayyiduna Mu'awiyah (may Allah be pleased with him) used to send some of his guards (to an unidentified place), saying: "Today is the day of Ashura'; the Prophet (peace and blessings be upon him) used to fast on it, and we are fasting, so whoever wishes may fast as well."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16951