Calvin and Hobbes by Bill Watterson for January 19, 1988
Max: Boy, Calvin takes that stuffed tiger everywhere he goes. Mom: Yeah, they're inseparable. Max: Do you worry about that? I mean, shouldn't he be playing with real friends? Mom: Oh, I think he will when he's ready. Didn't you ever have an imaginary friend? Max: Sometimes I think all my friends have been imaginary.