I remember reading about this claim about illegals voting a while ago, that's probably where Trump (who only knows what's on the Internet according to himself) got the claim.
The idea that so many people voted illegally? Trump didn’t make it up. Somebody else did.
The claim appears to have originated with Gregg Phillips, who identifies himself online as a resident of Austin, Texas, and the founder of an app for reporting voter fraud. A few days after the election, he tweeted that more than 3 million people had voted illegally, citing an “analysis” that he hasn’t released:
http://qz.com/846551/donald-trump-ed...news-movement/