From “Shoot Migrants’ Legs, Build Alligator Moats: Behind Trump’s Ideas for Border,” New York Times, October 2:

Privately, the president had often talked about fortifying a border wall with a water-filled trench, stocked with snakes or alligators, prompting aides to seek a cost estimate. He wanted the wall electrified, with spikes on top that could pierce human flesh. After publicly suggesting that soldiers shoot migrants if they threw rocks, the president backed off when his staff told him that was illegal. But later in a meeting, aides recalled, he suggested that they shoot migrants in the legs to slow them down. That’s not allowed either, they told him.

To adopt a Randian rhetorical trope: he got it from Israel, who got it from East Germany.