WASHINGTON (CNN) -- Former FBI Director James Comey wrote in a memo that President Donald Trump asked him to end the investigation of national security adviser Michael Flynn, according to a source familiar with the matter.

Comey was so appalled by the request that he wanted to document it, the source said. Comey shared it with FBI senior officials, according to the source.

Why did he do it?

Comey would write down everything that happened -- the good and the bad.

"Everything he could remember," the source said.

"You realize something momentous has happened and memories fade so he wanted to memorialize it at the earliest time," the source said. The source said it was not common practice for Comey to document conversations with senior officials unless he thought it was significant.

Conversations Comey had with Trump were documented by the former FBI director.

In one memo, Comey included a description of Trump talking to him about crowd size at the inauguration.

The source said Comey did not do this with President Barack Obama, in part, because he had fewer one-on-one conversations with him.

"There is no need to document the conversations with people who are truthful or situations that are routine. It's when you have situations that are not routine and people who are not truthful, you would write a memo to file," the source said. "There have been other occasions where he has done this but not everyday."

The conversation on February 14 was "more Flynn focused," not about the overall Russia investigation or the other associates under scrutiny.

The source said Comey would share the information with those close to him to use them, in part, as a sounding board.

The expectation is that the memos will be subpoenaed, according to a source familiar with the matter. The source says Comey wants to tell his story either in testimony to Congress, or in an interview or speech. A second source told CNN's Jake Tapper that Comey wants to testify and do so publicly.

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