"Angels & Daemons” (Season 7, Episode 13)
Sam and Callen are not only partners, but also carpool buddies. So, when Callen skipped on his morning ride it created a "thing" between them—causing Kensi to say they were arguing like a married couple.
Callen: You done with this thing?
Sam: If you say it’s not a thing, it’s not a thing.
Callen: It’s not a thing.
Sam: Unless you don’t realize it’s a thing.
Callen: Keep that up and you’re gonna have a thing.