What does Claudius say that shows he wants Hamlet to die?

You will likely find the answer to your question in act 4, scene 7. In this scene, Claudius actively plots with Laertes to ensure Hamlet's death. Here are two quotes that prove Claudius wants Hamlet to die:

If he be now returned,
As checking at his voyage, and that he means
No more to undertake it, I will work him
To an exploit, now ripe in my devise,
Under the which he shall not choose but fall.
And for his death no wind of blame shall breathe,
But even his mother shall uncharge the practice
And call it accident (act 4, scene 7).
 
 
When in your motion you are hot and dry,
As make your bouts more violent to that end,
And that he calls for drink, I’ll have prepared him
A chalice for the nonce, whereon but sipping,
If he by chance escape your venomed stuck,
Our purpose may hold there (act 4, scene 7).
 

In the first quote, Claudius tells Laertes that he has devised a plan ("exploit") that will ensure Hamlet's death. Claudius believes that his plan is so airtight even Hamlet's mother will not suspect foul play in the death of her son.
 
The second quote also supports the fact that Claudius wants Hamlet to die. Here, Claudius tells Laertes that, should Hamlet escape Laertes' poisoned sword, he has concocted a lethal potion for Hamlet to drink. These quotes show that Claudius is actively plotting to make sure that Hamlet dies in one way or another. Claudius's behavior in act 4, scene 7 demonstrates his manipulative and corrupt nature.