I am not 100% sure he said gross or operating but did say 14%.
I think it was during a bloomberg interview. I am not going to look through all the youtube vids looking for it.
He talked about taking lower margins in order to increase volume vs Model S.
It was probably operating margin. He's said multiple times that he expects Tesla long-term operating margins to be in the mid-teens. Gross margins long term will likely be in the mid-20% range.