Hi Pedro,
Create a validation rule on the Timesheet entity. Please see the attachment.
More examples of validation rule can be found : http://usermanual.clarizen.com/#business-rules
or https://app.clarizen.com/Clarizen/Pages/Admin/AdminMenu.aspx?pid=4&SA=1&CSRT=10257327607846313522
I also encourage you to attend one of our free QA webinars where you can find more information: http://www.clarizen.com/resources/online-demo.html#second