Reference for Rosh Hashanah 36:2
אמר רב פפא הכי קאמר בזמן שיש שלום יהיו לששון ולשמחה יש גזרת המלכות צום אין גזרת המלכות ואין שלום רצו מתענין רצו אין מתענין
If that i the case, the ninth of Ab also [should be optional]? - R'Papa replied: The ninth of Ab is in a different category, because several misfortunes happened on it, as a Master has said: On the ninth of Ab the Temple was destroyed both the first time and the second time, and Bethar was captured<span class="x" onmousemove="('comment',' In the war of Bar Cochba.');"><sup>4</sup></span> and the city [Jerusalem] was ploughed.<span class="x" onmousemove="('comment',' V. Ta'an. 20b.');"><sup>5</sup></span> It has been taught: R'Simeon said: There are four expositions among those given by R'Akiba with which I do not agree.