This adagio is the slow movement from Alessandro Marcello's oboe concerto in d minor (he is the elder brother of the other Marcello, Benedetto, well known for his Psaume XVIII).
Bach transcribed this concerto for keyboard when he was at Weimar, along with several other italian concertos.
This Adagio is a beautiful meditation, or perhaps deploration. I feel very clumsy while playing this kind of music, but it may be a good - if uncomfortable - exercise to venture sometimes outside one's comfort zone :-).