Who was the first person to knock cassius clay to the floor?

The first person to knock Cassius Clay, who later became known as Muhammad Ali, to the canvas was Sonny Liston. This occurred during their heavyweight championship fight on February 25, 1964, when Clay was still a young and relatively inexperienced boxer. Liston knocked Clay down in the fourth round, but Clay managed to recover and ultimately won the fight, claiming the heavyweight title.