'Trump's phrase "when the looting starts, the shooting starts" is an unattributed quote of Walter Headley, Miami's police chief in 1967. It was a threat to citizens who were upset that police had terrorized a black teenager by holding him over a bridge.' (https://twitter.com/mattsheffield/status/1266246092393336838)
The OP means that Trump didn't use quotation marks or mention that it was a quotation. (I myself didn't realize it was a quotation.) There is no attribution or quotation marks in the tweet.
I found the attribution of the quote very informative and am very surprised it is currently downvoted. My own summary of the tweets is also downvoted, not sure what I could/should add to it: https://news.ycombinator.com/item?id=23347395
I tried to neutrally summarize the tweets, in case people wanted to know what they said without clicking.
