From “Racism and Science Fiction” by Samuel R. Delany:
“In February 10, a month and a half before the March awards, in its partially completed state Nova had been purchased by Doubleday & Co. Three months after the awards banquet, in June, when it was done, with that first Nebula under my belt, I submitted Nova for serialization to the famous sf editor of Analog Magazine, John W. Campbell, Jr. Campbell rejected it, with a note and phone call to my agent explaining that he didn’t feel his readership would be able to relate to a black main character. That was one of my first direct encounters, as a professional writer, with the slippery and always commercialized form of liberal American prejudice: Campbell had nothing against my being black, you understand. (There reputedly exists a letter from him to horror writer Dean Koontz, from only a year or two later, in which Campbell argues in all seriousness that a technologically advanced black civilization is a social and a biological impossibility. . . .). No, perish the thought! Surely there was not a prejudiced bone in his body! It’s just that I had, by pure happenstance, chosen to write about someone whose mother was from Senegal (and whose father was from Norway), and it was the poor benighted readers, out there in America’s heartland, who, in 1967, would be too upset. . . .”

Originally published in Dark Matter edited by Sheree R. Thomas, Warner Books: New York, 2000. Republished in The New York Review of Science Fiction, Issue 120, August 1998.

Also mentioned in…

John W. Campbell’s Wikipedia Entry:
“On February 10, 1967, Campbell rejected Samuel R. Delany’s Nova a month before it was ultimately published, with a note and phone call to his agent explaining that he did not feel his readership “would be able to relate to a black main character”. There reputedly exists a letter from him to horror writer Dean Koontz in which Campbell argues that a technologically advanced black civilization would be a social and a biological impossibility.”

“Everything wrong with science fiction is John W. Campbell’s fault” by Alis Franklin
“Delany, in his own words about the incident mentioned above:
‘On February 10, a month and a half before the March awards, in its partially completed state Nova had been purchased by Doubleday & Co…'”

Partners in Wonder: Women and the Birth of Science Fiction, 1926-1965 by Eric Leif Davin, p21
“Samuel R. Delany, “Racism and Science Fiction,” in Sheree R. Thomas, Ed., Dark Matter: A Century of Speculative Fiction from the African Diaspora, Warner Books: N.Y., 2000, pp. 387-388 Delany also relays the rumor that author Dean Koontz has a letter from Campbell reflecting racist views. However, Delany does not claim to have read this letter and until Koontz produces it, this must remain, like Delany’s report of what Campbell supposedly told his agent, merely hearsay.”

Gender, Race, and American Science Fiction: Reflections on Fantastic Identities by Jason Haslam
“In some ways, this episode retells the actual events surrounding Samuel Delany’s attempt to publish Nova in Analog, edited by John W. Campbell. As Delany writes:
‘l submitted Nova for serialization to the famous sf editor of Analog Magazine, John W. Campbell, Jr. Campbell rejected it, with a note and phone call to my agent explaining that he didn’t feel his readership would be able to relate to a black main character…'”