The text doesn't actually tell us. We know that Iago wounds Cassio in the leg from behind, when Roderigo fails to kill him:
I know his gait; 'tis he. Villain, thou diest!
That thrust had been mine enemy indeed,
But that my coat is better than thou know'st;
I will make proof of thine.
O, I am slain!
[Iago from behind wounds Cassio in the leg, and exit.]
I am maim'd forever. Help, ho! Murder! Murder!
Roderigo is wrong, though. He isn't slain. He lies on the floor crying out for help, but no-one dares come in case he's a faker trying to set a trap so he can hurt them:
O wretched villain!
Two or three groan; it is a heavy night:
These may be counterfeits; let's think't unsafe
To come in to the cry without more help.
Nobody come? Then shall I bleed to death.
Roderigo lies there in the dark. Until Iago steps in with a light to see what is going on - and finishes the job:
O treacherous villains!
What are you there? Come in and give some help.
O, help me here!
That's one of them.
O murderous slave! O villain!
O damn'd Iago! O inhuman dog!
And that, stabbed in the dark, is the end of Roderigo.