What music is bach holding in his portrait?

The music Bach is holding in his portrait is the manuscript for his motet "Singet dem Herrn ein neues Lied" ("Sing to the Lord a new song"), BWV 225. This motet was composed in 1727 for the Leipzig Thomaskirche, where Bach was the Thomaskantor (music director) at the time. The manuscript is now housed in the Bach-Archiv in Leipzig, Germany.