As far as I know, RSBN has covered all of the Trump rallies for several years. I scrolled down their “Videos” page (that I linked) and found the earliest one (look for the preview photos that have Trump with a crowd behind him) ...
FULL Speech: Donald Trump Holds MASSIVE Rally in Beaumont, TX (11-14-15)
https://www.youtube.com/watch?v=Pv0iqg1PnSE [sound starts at 0:35]
<<>>
And earlier (hosted by Tea Party Patriots) ...
FULL Speech: Donald Trump At DC Rally Against Iran Nuclear Deal (9-9-15)
https://www.youtube.com/watch?v=8eTK3D42Wus [introduced as businessman Donald Trump]
Much appreciated.