Arthur 'involuntary astral projection while in a magical coma' fic
Read this one ages ago and can't find it again. Canon era. The premise is that Arthur gets hit by a spell which puts his physical body into a coma, but his soul/spirit is free to wander around (nobody can see or hear him like this). He follows Merlin around all day like this and eventually, unbeknownst to Merlin, discovers Merlin has magic.
Morgana has a crystal(?) which needs to be broken to return Arthur's soul to his body; Merlin sets off to find it with Arthur following him. Merlin succeeds but runs into trouble. Arthur wakes back up in his own body and immediately rushes off to go save Merlin. Fairly wholesome.