it first started in the real slim shady when em said " will smith dont need to cuss in his rhymes to sell records, well i do so **** him and **** you too". he did some other stuff like doing a stupid impression of Will at an awards ceremony.
Then on lost and found, a good few years later Will responded on "mr nice guy" and said. "Dissed by Eminem but did it bother him (yup), but he classy, Big Will just get another 20 mil, and walk right pass E."
nothing really came of it then.