Fans at Willie Nelson's Outlaw Music Festival Tour stop in Cincinnati, Ohio, on Friday (June 22) got a quite a surprise: Nelson invited Sturgill Simpson onstage for a duet on the song "Mammas Don't Let Your Babies Grow Up to Be Cowboys"

Readers can press play above to watch Nelson and Simpson's performance. The two exchange verses of the classic country song; the audience was clearly into the pairing, belting out the well-known chorus along with the men. Nelson first sang "Mammas" in 1978 with Waylon Jennings, for their Waylon & Willie album; the collaboration netted the pair a Grammys trophy.

"Mammas Don't Let Your Babies Grow Up to Be Cowboys" was written by Ed and Patsy Bruce, and originally recorded by Ed, for his self-titled 1976 album. Chris LeDoux also recorded a version of the song, released that same year.

As reported by Rolling Stone, Nelson and Simpson's collaboration during the show at Ohio's Riverbend Music Center didn't end with "Mammas." Simpson hung around onstage through the rest of Nelson's set and participated in a finale medley featuring, among other songs, "Will the Circle Be Unbroken" and "I Saw the Light."

Nelson's Outlaw Music Festival Tour is scheduled to run through Sept. 23. Simpson, among other artists, is accompanying the country legend at a number of the stops. More information about the tour can be found on Nelson's website.

See Photos of Willie Nelson + More Country Stars Who Have Served in the Military

Test Your Willie Nelson Knowledge!