The first half of the passacaglia's ostinato, which also serves as the fugue's main subject, was most probably taken from a short work by the French composer André Raison, "Christe: Trio en passacaille" from "Messe du deuxieme ton" of the "Premier livre d'orgue". It is possible that the second half of the ostinato was also taken from Raison, the bass line of "Christe: Trio en chaconne" of "Messe du sixieme ton" of the same publication is very similar. See "Example 1" for Bach's and Raison's themes.