During a White House press conference on September 23, Trump deflected from a question on whether he would accept the outcome of the election.

SPEAKER: ..Do you commit to making sure that there’s a peaceful transferal of power?

 

TRUMP: We want to have… Get rid of the ballots and we’ll have a very peaceful… There won’t be a transfer frankly, there’ll be a continuation. The ballots are out of control. You know it, and you know who knows it better than anybody else? The Democrats know it better than anybody else.

Even up to the election itself, he refused to confirm that he would accept any outcome other than his own victory.

Forbes – Trump Won’t Commit To Peaceful Power Transfer, Says ‘Get Rid Of The Ballots’ For ‘Continuation’ Of His Presidency
NBC News – Trump on peaceful transition if he loses: ‘Get rid of the ballots’ and ‘there won’t be a transfer’
Rev – White House Press Briefing, September 23, Transcript