There's a fic I read recently that I've been trying to find again. It was very short, just a one-shot/drabble, and I think it was posted on Tumblr. FOUND AND LINKED IN THE COMMENTS.

It took place after Dean made it out of Purgatory, but before Cas. Sam's POV, implied Dean/Cas. Basically, the fic was Sam noticing Dean's nightmares changing after Purgatory; IIRC, his nightmares post-Hell/pre-Purgatory led him to say things in his sleep occasionally, but only a word or two, and not often, and then one night not long after he made it out Sam woke up because he was speaking in full sentences, and to Cas.