He heard Muawiya bin Abu Sufyan, on the day of Ashura during the year he performed Hajj, say from the pulpit, "People of Medina, where are your scholars? I heard the Messenger of Allah ﷺ say about this day, 'This is the day of Ashura. Its fasting has not been prescribed upon you, but I am fasting. Whoever wishes may fast, and whoever wishes may break his fast.'"
أنه سمع معاوية بن أبي سفيان يوم عاشوراء، عام حج وهو على المنبر يقول: يا أهل المدينة، أين علماؤكم؟ سمعت رسول الله ﷺ يقول لهذا اليوم: هذا يوم عاشوراء، ولم يكتب عليكم صيامه وأنا صائم، فمن شاء فليصم، ومن شاء فليفطر.
Malik narrated it in al-Siyam (34), from Ibn Shihab, from Humayd bin Abdul-Rahman bin Awf, and he related it.Bukhari narrated it in al-Sawm (2003), and Muslim in al-Siyam (1129), both of them through the route of Malik, with the same chain.