Sahih Muslim 1129
Sahih Muslim · Book 13: The Book of Fasting · Fasting on the day of Ashura'
This hadith has been narrated on the authority of Zuhri with the same chain of transmitters that be heard Allah's Apostle (ﷺ) as saying on a similar day:
وَحَدَّثَنَا ابْنُ أَبِي عُمَرَ ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ ، عَنِ الزُّهْرِيِّ ، بِهَذَا الإِسْنَادِ سَمِعَ النَّبِيَّ صلى الله عليه وسلم يَقُولُ فِي مِثْلِ هَذَا الْيَوْمِ " إِنِّي صَائِمٌ فَمَنْ شَاءَ أَنْ يَصُومَ فَلْيَصُمْ " . وَلَمْ يَذْكُرْ بَاقِيَ حَدِيثِ مَالِكٍ وَيُونُسَ .
I am fasting today, so he who wishes to observe fast should do so; but he did not make mention of the rest of the hadith.