All Questions

Let us say a forcing $P$ is proper for a class of models $\mathcal C$, if for large enough regular $\theta$ and $M \prec H_\theta$ in $\mathcal C$ with $P \in M$, every $p \in P \cap M$ can be ...

Without success, I have been trying to find who was the first to prove the folklore result that any finite support iteration of non-trivial posets adds Cohen reals at limits steps. Does anybody know?