It is not an "unnamed source," it's New York Magazine, the journalist Julie Baumgold wrote a Trump article (Nov 9, 1992) and she was quoting Trump directly. This is how she transcribed the conversation. She was in the car with the two men when he said it. 

    "Phillip Johnson looks at his watch. He is going to Berlin this evening to design a building at Checkpoint Charlie. Trump is talking about women and says, "You have to treat 'em like s---"

    "You'd make a good mafioso," says Johnson

    "One of the greatest," says Donald as the car glides through the protected pinelands into the corrupted city."

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You can read the article yourself on Google books, at https://books.google.com/books?id=BeUCAAAAMBAJ&lpg=PP1&pg=PA43#v=onepage&q&f=false

Now, given it took me exactly five minutes on Google to find the source, I can't help but wonder what your motivation is NOT to look it up? Why on earth would you choose to stay oblivious to the truth?