Sahih Muslim Book 6, Hadith Number 2517.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
This Hadith has been narrated on the authority of Zuhri with the same chain of transmitters that be heard Allah’s Apostle (may peace be upon him) as saying on a similar day: 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.