Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2514
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 hence. forth neither commanded us nor forbade us, nor was he so particular about it.