Suing My Ex for Slander?

I just got a call from a friend today, and she was telling me how my ex came into where she works and was talking to a coworker of hers and telling him that we are still together and having all the time (mind you I am VERY happily married) and that even though I am still with my husband I go to his house to have with him. This is a 150% lie and I am tired of him spreading rumors and lies about me. I don't have the proof for all of it but I KNOW he is doing it. I am not going to allow him to ruin my marriage. Can I sue him for slander?