I think this fic read from Sam's POV, and it was set some time after 5xo4 'The End'. It could either be a longer fic or part of a series of fics, I'm not sure which.

The thing sticking in my mind is a scene where Dean is on the phone to Cas and sarcastically asks something along the lines of, "What are you wearing, baby?" He then goes on to explain that it's a joke and that yes, he knows Cas is wearing his suit and trenchcoat as usual. Meanwhile Sam's wondering when Dean got so comfortable with Castiel that he could mess around with him like that.

Any help very much appreciated - I know it's not much to go on.

Thanks!

ETA: Found, thanks to the wonderful hugglewolf - it's The Mountains in Reply by unoshot