I think a guy called Kent Green knocked Clay out. I'm not sure how old Clay was at the time. Obviously young.
In is biography he did mention his only stoppage defeat as an amateur was the day after he lost his virginity to a *****!
Kent Green Tony Madigan Jimmy Eliis Amos Johnson Green knocked him down a couple of times and ref stopped it. His own corner threw in towel vs Madigan, who he beat in the Olympics.