This source, who has knowledge of the legal issues surrounding Comey, said he "was in the middle of an ongoing investigation" and "everybody would lawyer up and head for the hills."
The source said Comey did the right thing: made notes of his conversation and sent it to another member of his leadership team.
"It's textbook," the source said. He also adds that Comey was not required to inform other parts of the Justice Department.
The New York Times first reported on the memo and CNN has confirmed the details of its content.
In terms of whether Trump actually meant to obstruct an investigation, he says that "motive always matters" especially since this is a case of "he said, he said." But he worries about Trump in this instance and says "he better hope there aren't any tapes."
This source predicted it's likely that there are other memos to come.
He said once the President asks to quash something, the prosecutor will believe there is something to hide.
The source said he thinks the White House spin is laughable, adding that folks in the administration are upset and anxious.
He suggests a better move for the President would be to come out and say "I want to get to the bottom of this."
Comey's only weakness, he says, was his response to Trump in which Comey told Trump that Flynn was indeed a "good guy," according to The New York Times' description of the memo.
"It makes you think he wasn't sure of the intent of the President's remarks," the source said.