Just in case it comes up again and anyone needs it, here’s the first GOP debate in Cleveland on August 6, 2015 in which every nominee on the stage except Trump silently pledged to support the eventual GOP nominee:
Starts at 03:38:
BAIER: Gentlemen, we know how much you love hand-raising questions. So we promise, this is the only one tonight: the only one. Is there anyone on stage, and can I see hands, who is unwilling tonight to pledge your support to the eventual nominee of the Republican party and pledge to not run an independent campaign against that person.
Again, we’re looking for you to raise your hand now — raise your hand now if you won’t make that pledge tonight. […]
A little less than a month later, Trump signed the pledge.