I believe this evidence should come from the text itself.
In order to show that you understand something that you've just read, you will have to mention specific details from the text itself to prove and support your understanding. Other sources can just help you provide additional details to your claims, but ultimately, it should be the original text that you cite and talk about in order to show your analysis and interpretation. You should use details from the text to show whether you think it is right or wrong and show your understanding.