President Donald Trump hit back at former Secretary of State Rex Tillerson on Friday after Tillerson spoke out about his time in the Trump administration.

What's the history?

On Friday, Tillerson told "CBS This Morning" that he found working with Trump to be challenging because he did not believe the president to be very professional.

Tillerson also added that Trump didn't care much for reading and revealed that he had to explain to the president on several occasions that some of the commander in chief's proposals were not exactly legal.

Tillerson also once purportedly — and famously — called Trump a "moron" after a meeting at the Pentagon.

In March, Trump fired Tillerson and replaced him with Secretary of State Mike Pompeo.

WATCH: Former Secretary of State Rex Tillerson is speaking publicly about what led to his firing in March by President Trump.

CBS News political contributor @bobschieffer interviewed Tillerson in Houston last night, at a dinner to benefit MD Anderson Cancer Center. pic.twitter.com/47qDqcsrMs
— CBS This Morning (@CBSThisMorning) December 7, 2018

What did Trump say?

Later Friday, Trump addressed Tillerson's remarks on Twitter. He wrote, "Mike Pompeo is doing a great job, I am very proud of him."

"His predecessor, Rex Tillerson, didn't have the mental capacity needed," Trump continued. "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!"

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!
— Donald J. Trump (@realDonaldTrump) December 7, 2018