Reference for Rosh Hashanah 4:13
לא ס"ד דכתיב (דברים א, ד) אחרי הכותו את סיחון וכי נח נפשיה דאהרן אכתי הוה סיחון קיים דכתיב
so here, [I may say that the occurrence of] the expression 'fortieth year' in the two contexts provides us with a gezerah shawah, [showing that] just as in the one case<span class="x" onmousemove="('comment',' The death of Aaron.');"><sup>19</sup></span> [the date is reckoned] from the Exodus, so in the other case<span class="x" onmousemove="('comment',' The address of Moses.');"><sup>20</sup></span> also. But how do you know that [in respect of these two incidents] that of Ab was prior? Perhaps that of Shebat was prior?<span class="x" onmousemove="('comment',' I.e., the address of Moses was prior to the death of Aaron, the fortieth year having commenced with the Tishri preceding Moses' address.');"><sup>21</sup></span> - Do not imagine such a thing. For it is written [in connection with the latter], 'After he had smitten Sihon';<span class="x" onmousemove="('comment',' Deut. I, 4.');"><sup>22</sup></span> and when Aaron died Sihon was still alive, as it is written