Reference for Taanit 35:12

וכן אמר באלי אמר רבי חייא בר אבא א"ר יוחנן הלכה כרבי יוסי א"ל רבי חייא לבאלי אסברא לך כי אמר ר' יוחנן הלכה כרבי יוסי אדלא להתענאה

Now that you have arrived at this conclusion the same will apply also to the twenty-ninth Adar because should it ever come to pass that the twenty-eighth Adar be abolished as a festive day, even then the twenty-ninth would be forbidden seeing that it is the day before the Daily offering was established. It has been taught: R'Hiyya B'Asi said in the name of Rab, the halachah is in accordance with the view of R'Jose.<span class="x" onmousemove="('comment',' Who holds that the restriction applies both to the day before and the day after the festive days.');">⁹</span>

Jastrow