Sahih Muslim Book 6, Hadith Number 2519.
Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).
This Hadith has been narrated by Ibn Bishr with the same chain of transmitters (but with a slight variation) that he (the Holy Prophet) inquired of them (Jews) about it (fasting on the day of ‘Ashura).