Sahih Muslim Book 6, Hadith Number 2514.

Chapter : Fasting on the day of ‘Ashura (1Oth of Muharram).

Jabir b Samura reported that the Messenger of Allah (may peace be upon him) commanded us to observe fast on the day of Ashura and exhorted us to do it and was particular about it But when (fasting) in Ramadan was made obligatory, he henceforth neither commanded us nor forbade us, nor was he so particular about it.