He heard the Saviour Christ, the one who died, buried, resurrected, ascended and sitting in the right hand of God. Well, Paul have heard the preaching of Stephen.
Acts 7:58-60
58 And cast him out of the city, and stoned him: and the witnesses laid down their clothes at a young man's feet, whose name was Saul.
59 And they stoned Stephen, calling upon God, and saying, Lord Jesus, receive my spirit.
60 And he kneeled down, and cried with a loud voice, Lord, lay not this sin to their charge. And when he had said this, he fell asleep