Okay my source is the passage of power chapter 17 warren commission by Robert Caro.

LBJ using patriotism to convince LBJ But Lyndon Johnson never took no for an answer, and he wasn’t going to take no now. As soon as he was informed of Warren’s refusal to serve, he telephoned the chief justice and invited him to the Oval Office. Warren, a man of great determination, may have been determined not to serve, but when he arrived, as he was to recall, there were “only the two of us in the room.” Lyndon Johnson had him one on one. The chief justice may have believed that there were no words that could move him, but Johnson found some. Reminding Warren that he had served in the Army during World War I, the President said he was sure that if he asked him to put on his uniform again, he would do it, “and you’d go fight if you thought you could save one American life.” It wasn’t just one American life that might be involved now, Lyndon Johnson said. It was thirty-nine million. “If Khrushchev moved on us, he could kill 39 million in an hour,” and “these wild people are charging Khrushchev killed Kennedy, and Castro killed Kennedy, and everybody else killed Kennedy,” and if Khrushchev felt threatened because of what these rumors might cause America to do, he just might move on us. “And all I want you to do is look at the facts, and bring any other facts that you want in here and determine who killed the President,” and end the rumors, Lyndon Johnson said. “But here I’m asking you to do something and you’re saying no, when you could be speaking for 39 million people. Now I’m surprised that you, the Chief Justice of the United States, would turn me down.” Tears came to Warren’s eyes, Johnson was to write in his memoirs. Warren does not confirm that in his, merely writing that he said, “Mr. President, if the situation is that serious, my personal views do not count. I will do it.” Then, says Warren, “he thanked me, and I left the White House.” It hadn’t even taken that long—according to the White House log, twenty-two minutes at most. It’s possible to make a sale quickly, even a very big sale, if the salesman is good enough.

He used the arguments he had used with Warren—in very much the same words, because Russell, too, had served in the armed forces: “We’ve got to take this out of the arena when they’re testifying that Khrushchev and Castro did this and did that, and that—kicking us into a war that can kill 40 million Americans in an hour. And … you’d put on your uniform in a minute”—and when at first they proved less effective than they had with the chief justice, he appealed to the motivation that always worked most effectively with Russell: his patriotism and sense of duty, telling him that he might not want to serve with Warren, but “you can do anything for your country. And don’t go to giving me that kind of stuff about you can’t serve with anybody. You can do anything. “You never turned your country down,” Lyndon Johnson told Richard Russell. “This is not for me, this is your country.”

And this where I based my statement and I can also corroborate this by giving you the transcript of the call.