In Nov. 2015, DT approved of a database of US Muslims.
Right now he is only calling for limits on Muslims entering the country, but he has already indicated an openness to registering all Muslims here, which would include citizens.
http://www.independent.co.uk/news/world/americas/doald-trump-muslim-register-database-protest-madeleine-albright-mayim-bialik-white-house-refugee-ban-a7546896.html
The seed of the idea was apparently planted by a reporter's question, who asked "is there going to be a database that tracks the Muslims here in this country?"
"There should be a lot of systems, beyond database, we should have a lot of systems, and today you can do it," the President replied, speaking in November 2015 when he was still one of several in the race for the Republican nomination.
But when asked specifically about a database, he continued: "I would certainly implement that." Asked how such a database could work in practice, he simply replied "good management."
He has since variously said that "I certainly wouldn't want to do it but we have to be vigilant," and later claimed "I didn't say that. I never said that."
Carl Higbie, a former Navy SEAL and spokesperson for the Great America PAC for Donald Trump, cited Japanese internment camps during World War II as "precedent" for the database.