President Donald Trump spent Tuesday ripping into a book written by former staffer Cliff Sims about his experiences in the administration.

“A low level staffer that I hardly knew named Cliff Sims wrote yet another boring book based on made up stories and fiction,” Trump wrote. “He pretended to be an insider when in fact he was nothing more than a gofer.”

Sims joined the Trump campaign in 2016 and went to the West Wing as a mid-level communications aide for the president.

His book Team of Vipers takes aim at the people in Trump’s White House by leaking details of the first 500 days of the administration.

“I was there. This is what I saw,” Sims writes in the introduction. “And, unlike the many leakers in the White House, I have put my name on it.”

The book includes details several conversations he had with the president and features several pictures of the two of them together.

In recent days, Sims has been on television promoting the book with CBS late night host Stephen Colbert, MSNBC’s “Morning Joe,” ABC’s “The View,” and through an interview with ABC host George Stephanopoulos.

Trump reminded Sims that he signed a non-disclosure agreement in the White House.

“He signed a non-disclosure agreement,” he wrote. “He is a mess!”