Gale Sayers, the Chicago Bears halfback whose 1970 autobiography, I Am Third, formed the basis for the hit TV movie Brian’s Song, has died at 77, ESPN reports.

Sayers played seven seasons with the Bears, appearing in four Pro Bowls and leading the league in rushing yards twice. He was inducted into the Pro Football Hall of Fame in 1977.

But many know him chiefly as the author of I Am Third, a book co-authored with sportswriter Al Silverman. It told the story of his life, including his friendship with teammate Brian Piccolo, who died of cancer at the age of 26. The title comes from a line in the memoir: “The Lord is first, my friends are second, and I am third.”

“As much as they cut into this man, as much as he was inflicted with terrible pain and discomfort, as much as he suffered because of this wicked disease that struck him like a thunderbolt flashing out of a clear sky…as much as he was faced with all these tortures, his spirit would not be destroyed,” Sayers wrote in the book. “That was the beautiful nature of Brian Piccolo.”

The book was adapted into a 1971 ABC television movie, Brian’s Song, which became one of the most iconic films of the decade. Piccolo was played by James Caan, and Sayers by Billy Dee Williams.

“My heart is broken over the loss of my dear friend, Gale Sayers,” Williams wrote on Twitter. “Portraying Gale in Brian’s Song was a true honor and one of the [highlights] of my career. He was an extraordinary human being with the kindest heart.”

Michael Schaub is a Texas-based journalist and regular contributor to NPR.