He did write this on his twitter feed[archived]
> The concept of global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive.
See also politifact: Yes, Donald Trump did call climate change a Chinese hoax. He later claimed that this was a joke:
> Well, I think the climate change is just a very, very expensive form of tax. A lot of people are making a lot of money. I know much about climate change. I'd be — received environmental awards. And I often joke that this is done for the benefit of China. Obviously, I joke. But this is done for the benefit of China, because China does not do anything to help climate change.