Ah I see what you mean. I've never considered that scale to be Re as that would be pronounced incorrectly. It has to be Ray.
So the note must be E (from A, B, C, D E, F, G)
And the R can, therefore, only be hinted at from the word Retain
( As an aside, I've just watched my typing get auto typoed by something !)
But even then if one used Re to be a note, it isn't being retained by Bach since retained means continued to have, and Bach never had Re to retain in the first place. He was given the note.