for proper reference.

What did the Prophet Muhammad (peace be upon him) do when fasting in Ramadan was made obligatory?
 This hadith has been narrated from Jarir on the authority of A'mash with the same chain of transmitters and he said (these words with a little bit of variation from the previous hadith):                      When (fasting) in Ramadan was (made) obligatory, he aban- doned it (the practice of observing fast on Ashura).