Here are various sources:

http://www.politifact.com/truth-o-meter/statements/2015/nov/23/donald-trump/trump-tweet-blacks-white-homicide-victims/

http://www.telegraph.co.uk/news/worldnews/donald-trump/12012107/Donald-Trump-retweets-inaccurate-statistics-about-black-murders.html

http://www.nydailynews.com/news/politics/fact-checking-donald-trump-questionable-statistics-tweet-article-1.2443285

He tweeted out fake statistics by a white supremacist group: imagined in their tiny racist minds in order to justify violence against black people.

When a Trump supporter dares ask you for an example of Trump being racist - here it is. Lying about another race to 1) make them seem worse and 2) to be violent against them is like, the most racist thing you can do with your speech.