Excerpted from The Dodgers: 12 Years of Dodgers Baseball by Glenn Stout:
"Sophomore year, Koufax returned to Cinncinnati with a $20,000 Dodger offer and a dilemma. "He wanted to know what I thought, because, you know, Jews don't do that," Rothenberg remembered. I said, "Sandy, if these guys think you're that good that they're offering you that kind of money, take it. You can always go back to college." That was the last time I talked to him. Then he was just gone.
The day he signed with the home team, Marv Raab saw him standing outside Dubrow's Cafeteria in Brooklyn, a lanky kid in a Lafayette sweatshirt, telling the world, "I just signed with the Dodgers!" He seemed as surprised as everyone else."
The same quote from Marv Raab is also excerpted in Sandy Koufax: A Lefty's Legacy by Jane Leavy, though I'm not sure which book came out first.