The appearance from the former Late Night and Late Show host offered a reunion with bandleader Paul Shaffer, who is sitting in with his World’s Most Dangerous Band on The Tonight Show this week. The Roots, Fallon’s usual band, are off for the week as they prepare for Friday’s SNL50: The Homecoming Concert on Peacock.

Feb. 11 (UPI) -- David Letterman joined Jimmy Fallon during his monologue on The Tonight Show Monday. "Is this the 23rd hour of the Today show?" asked the former Late Show host, feigning confusion, after arriving on stage .

Letterman began his late night TV career at NBC, where he hosted the groundbreaking “Late Night With David Letterman” from 1982 to 1993. The show followed Johnny Carson’s “Tonight Show.” When Carson announced in 1991 that he would retire the following year, it was widely expected that Letterman would take over the earlier slot.

While Paul Shaffer filled in the for The Roots on “The Tonight Show Starring Jimmy Fallon,” he was reunited with a surprise guest: David Letterman!

Around the 3:47 mark of Jimmy Fallon's monologue on Monday's The Tonight Show, David Letterman came out of retirement to crash the party. "Is this the 23rd hour of the Today Show?" he quipped, before reuniting with former music director Paul Shaffer,