U.S. President Donald Trump took to Twitter on Friday to attack former Secretary of State Rex Tillerson after he said Trump asked him to do things that were against the law.

Trump praised the man now in the post, Mike Pompeo, and went on to say Tillerson lacked the capabilities to be successful at the job.

"Mike Pompeo is doing a great job, I am very proud of him. His predecessor, Rex Tillerson, didn’t have the mental capacity needed. He was dumb as a rock and I couldn’t get rid of him fast enough. He was lazy as hell. Now it is a whole new ballgame, great spirit at State!" Trump said in a tweet.

The tweet follows remarks made by Tillerson at a fundraiser for the MD Anderson Cancer Center in Houston on Thursday night, in which he said Trump would often tell him to do things in ways that would violate the law.

"When the President would say, 'Here's what I want to do and here's how I want to do it.' And I'd have to say to him, 'Well Mr. President, I understand what you want to do, but you can't do it that way. It violates the law,'" Tillerson said, according to video of the event aired by CBS News.

He described Trump as someone "who is pretty undisciplined, doesn't like to read, doesn't read briefing reports" and said that he "acts on his instincts."

Tillerson, a former chief executive at Exxon Mobil, was fired by Trump after 14 months. He said he never met Trump before the day he was nominated.

According to The Houston Chronicle, Tillerson also criticized Twitter, saying that the short attention span of Americans have led them to care little about knowing the issues.