Nun gut, wer bist du denn?
Ein Teil von jener Kraft, Die stets das Böse will und stets das Gute schafft.
The line comes shortly after Mephistopheles appears in Faust's study, having followed him in as a black poodle. To see this scene in context
The source in context in Faust (English)(with an 1808 translation).
Who then art thou?
Part of that power which still Produceth good, whilst ever scheming ill.