From the book Wild and Crazy Guys by Nick de Semlyen (as reported in People magazine):
Bill Murray on punching Chevy Chase in the face.
On February 18, 1978, on the set of Saturday Night Live, everyone was still fuming that Chevy Chase had quit the show for a better paying gig, but had returned as a guest host, de Semlyen explains. Fueled by competitiveness and mutual dislike, a heated conversation between Chase (a known “provocateur”) and Murry turned violent when Murray punched Chase.
“It was a huge altercation,” director John Landis, who witnessed the event, told de Semlyen.