During his campaign, Trump has said this:
I've already said he is very much of a leader. The man has very strong control over his country.
At the same time, he criticized the Russian system, while praising Putin over Obama:
Now it's a very different system, and I don't happen to like the system, but certainly in that system he's been a leader, far more than our president has been a leader.
Trump has confirmed this position ("he's a better leader than Obama") in later interviews, while rejecting the claim that he said that Putin is "a great leader of his country".
He's also called him a "tough guy", "brilliant", said that "at least he’s a leader, unlike what we have in this country"and "I’ve always had a good feeling about him", etc.
Trump also said:
I said, I don’t admire him. I said he was a strong leader, which he is. I mean, he might be bad, he might be good. But he’s a strong leader.
In an interview in 2013, Trump has also said:
And it really makes [Putin] look like a great leader, frankly
So while I wasn't able to find the exact phrase "[is a] great leader", it seems like an acceptable paraphrase of positive references to "strong leader", "very much of a leader", or "[looks] like a great leader", which Trump did use.