This is what the media is trying to connect Trump's tweet to:
"The "when the looting starts, the shooting starts" quote, it was reported, was borrowed from former Miami Police Chief Walter Headley, a highly-controversial figure whose stop-and-frisk policies were blamed for repeatedly igniting tensions between the African-American community and law enforcement in the coastal city.
Speaking at a press conference following a holiday weekend in 1967, Headley, who had been chief for 19 years, suggested he did not have any riots because they had made it known it would be met with deadly force.
"We haven't had any serious problems with civil uprising and rooting because I've let the word filter down that when the looting starts, the shooting starts," he said. "These are my orders: Not three days after, but now."
meaww.com/walter-headley-donald-trump-when-the-looting-starts-shooting-starts-miami-police-chief-1968-riots