What assembly is ValidationRule in?

I'm trying to build a validation rule for a C# WPF application by creating a class that implements ValidationRule. My problem is that I can't find the System.Windows.Controls assembly which - acconding to the documen开发者_开发知识库tation - contains it. Does anybody know the right assembly in the 3.5 runtime ? Thanks

Namespace: System.Windows.Controls
Assembly: PresentationFramework (in PresentationFramework.dll

The documentation is your friend. (Namespace != assembly)