On Thursday, Rex Tillerson, former Secretary of State and ExxonMobil chief executive sat down with CBS News. In the interview, he said president Donald Trump was “pretty undisciplined” and said they did not share a “common value system.”

It was also reported that during his time in the administration Tillerson called Trump “a moron.” The relationship was rocky. At first, it started off with the president asking Tillerson to serve as the nation’s top diplomat on the first day they met. Tillerson would say that the job “was challenging for me coming from the disciplined, highly process-oriented ExxonMobil Corporation to go to work for a man who is pretty undisciplined, doesn’t — doesn’t like to read, doesn’t read briefing reports, doesn’t — doesn’t like to get into the details of a lot of things but rather just kind of says, look, this is what I believe and you can try to convince me otherwise, but most of the time you’re not going to do that.”

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He’d also add that “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, it violates the treaty, you know.’”

On Friday, the president fired back. He took to Twitter and called Tillerson “dumb as a rock” and “lazy as hell.” The full tweet reads,  “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!”

The news came hours before John Kelly, chief of state, announced that he’d leave the administration at the end of the year.