Wait a second... Further down into article we have now discovered Trump's source. His source is a facebook post.
"The Republican presidential nominee is likely referring to an allegation from a woman who posted on Facebook this week that she voted for Republicans up and down the ballot, though the summary of her votes showed that the Hillary Clinton-Tim Kaine ticket was checked, too.
“I tried to go back and change and could not get it to work,” the woman, who voted in Texas, said. “I asked for help from one of the workers and she couldn't get it to go back either. It took a second election person to get the machine to where I could correct the vote to a straight ticket.”
The woman urged voters to “double check your selections before you cast your vote!”
According to election officials, however, the problem was likely a “voter error,” not a sign of election rigging."