How to legally proof that a user looked at some content?

I'm currently making a website where I'm referring the user to some of my affiliates and I can't track it with an affiliate link. So I need a way to legally proof to my affiliates that I referred the user. My first thought was to hash a token with the user's password and the affiliate link and store it in a database that way I wouldn't be able to manipulate with who was referred and not, but I don't want to prompt the user for their password before each referral. Do you have a way that I can track if the user was referred by my site without me being able to manipulate with who have been referred and who hasn't?