Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2373 (DarusSalam) ⓘ

It was narrated that Humaid bin 'Abdur-Rahman bin 'Awf said: "I heard Mu'awiyah say on the day of 'Ashura when he was on the Mindar: O people of Al-Madinah, where are your scholars? I heard the Messenger of Allah say on this day: "I am fasting, so whoever wants to fast let him do so."

أَخْبَرَنَا قُتَيْبَةُ، عَنْ سُفْيَانَ، عَنِ الزُّهْرِيِّ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ، قَالَ سَمِعْتُ مُعَاوِيَةَ، يَوْمَ عَاشُورَاءَ وَهُوَ عَلَى الْمِنْبَرِ يَقُولُ يَا أَهْلَ الْمَدِينَةِ أَيْنَ عُلَمَاؤُكُمْ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ فِي هَذَا الْيَوْمِ ‏ "‏ إِنِّي صَائِمٌ فَمَنْ شَاءَ أَنْ يَصُومَ فَلْيَصُمْ ‏" ‏ ‏.‏