X-Men: The Early Years #6

Xavier reads a news story about Namor and wonders if he is a possible mutant. Magneto comes to the same conclusion and invades the Sub-Mariner's realm with an invitation to join his Brotherhood. Enchanted by the Scarlet Witch, Namor agrees. The X-Men attack and, during the ensuing battle, Namor decides not to join either team, mutant or no.