Authentication in Web Services

 Hi
What I need here is a solution
 
My Web Application will be talking to a Web Service i am building
The Web Service will handle all Business Logic for the Web Application
so the Web Service has to make sure for every request it recieves the user is a valid user
 
Let me explain a bit more:
 
User Will login in the Web App ( what the Web App will do is to call the Web Service to check if the user is valid or not)
Now if the user try to make another request (for example a payment) then the Web App will call the Web Service again and ask it to do the payment
in this case i want the Web Service to Confirm that user trying to make a payment is already valid user, how to do that?
do i have to check for username and password for every request?
what is the best solution for that?
 
Thanks
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