You may be thinking of Love of My Life -- he played harp on that song.
Contrary to popular belief, Brian May is not a harpist and can not play the instrument.
For 'Love Of My Life' Brian had to play each chord individually, recording the track chord by chord, and finally stitching all the small fragments together to create some semblence of a unified piece.
It works well, but please do not invite him to play the instrument live, as he just could not do it!
"Listen to them. Children of the night. What music they make."