Another classic piece. The original music of the choral is by Hans Leo Hassler, written around 1600 as a secular love song, "Mein G'müt ist mir verwirret" ("I'm confused", translated very freely). Johann Sebastian Bach used parts of the hymn in his St Matthew Passion (Matthäuspassion, BWV 244) and for other of his works. Believe it or not, Paul Simon's "American Tune" is mainly based on this one.
F (J.S. Bach)
Transposed from a piano lead sheet and strongly simplified. To not overcomplicate things, I left out some of the typical Bach bassruns.