Stocks Validation in Sales Orders

Hi experts,

I have the following requirement and I would like to some advice about how I should to face it.

For specific sale documents, I need a validation that doesn't let save orders if there is any position with material who hasn't stock.

Maybe I could make this validation by applying a user exit checking confirmed quantity and then using a pop up message error who doesn´t let to save the order.

Does any have any idea? or What user exit can i use?

I'll appreciate a lot any suggestion.

Best regards

José