Trump repeatedly claimed that the 2016 election was rigged, and cast doubt on whether he would accept the results as legitimate. He reiterated this during a debate with Hillary Clinton when asked if he maintained his view on the election.

I will look at it at the time. I’m not looking at anything now; I will look at it at the time. What I’ve seen – what I’ve seen is so bad. First of all, the media is so dishonest and so corrupt, and the pile-on is so amazing. The New York Times actually wrote an article about it, but they don’t even care. It’s so dishonest, and they’ve poisoned the minds of the voters. But unfortunately for them, I think the voters are seeing through it.

…

What I’m saying now is that I will tell you at the time. I will keep you in suspense, OK?

Trump later blamed his loss of the popular vote on mass fraud.

Politico – Trump still won’t commit to accepting election results
Politifact – Donald Trump’s Pants on Fire claim of ‘large scale voter fraud’