Season 4 finale, in the angels’ prison, when Castiel is going to bust Dean out. He appears without warning, shoves Dean up against a wall, covers his mouth, and pulls out a knife. Dean, who has been hunted, fought, beaten, and nearly killed for his whole life—this guy, Dean, just looks at the knife, looks at Cas, and nods.
YES! That fucking nod, ugh, it’s killing me. Destiel is canon.