I just reviewed that remark on Snopes and it seems that he didn’t say that. It first surfaced in a 1995 book. BUT, Snopes goes on and on and on with LBJ’s many racist remarks about black people and then goes to great pains to prove he wasn’t a racist. It’s funny contortions that they twist themselves into.
He probably didn't say it "on the record", but I have no doubt he said it. Probably yelled it from the crapper seat.