David Letterman
Ball State alumnus David Letterman, host of CBS's "Late Show," at Ball State University in Muncie, Indiana Nov. 26, 2012. Reuters/Chris Bergin

David Letterman, who announced his retirement Thursday, may have made the decision as a result of high ratings for the newly appointed “Tonight Show” host, Jimmy Fallon, a source closely connected to “The Late Show” told Fox411.

“David is older now; he had heart surgery some time ago and sees both Jimmy Kimmel [of ABC’s “Jimmy Kimmel Live!”] as well as the more formidable Jimmy Fallon now gaining on him,” the insider told FOX411. “Fallon is getting record ratings, so Letterman will segue out while he’s still on top. It’s dignified. If Fallon weren’t so hot, he might have stuck around longer.” Letterman has reportedly been averaging 2.85 million viewers, while Fallon has been receiving 4.3 million an episode.

Fallon addressed Letterman’s retirement on his show Friday, recalling the time when he was guest hosting for “The Late Show,” saying: "It's hard to imagine late-night TV without him, because he revolutionized it." He reportedly said that Letterman was “one in a million.”

Fallon reportedly presented in Letterman style the top 10 reasons why “The Late Show” host was retiring. "He made it what it is today. And after I heard the news, I kind of started thinking about reasons why Letterman is retiring," he said that the first reason is “Jimmy Fallon is stealing his bits.”

According to The Hollywood Reporter, Fallon’s top 10 reasons why Letterman is retiring are as follows:

10. Wants to quit while he's still able to compete on "Dancing with the Stars."

9. Made millions selling crack to Rob Ford.

8. Time slot was getting too "Jimmyesque."

7. He's not retiring – he's "consciously unhosting."

6. Stop! Hammock time!

5. Hillary/Letterman 2016!

4. After 31 years, finally got through to the Obamacare website!

3. One word: E-Looprah

2. Jay Leno needs a partner for next season's 'Amazing Race.'

1. Jimmy Fallon is stealing his bits.