First, from the horse's mouth:
then these other sources:
http://www.focus-fen.net/news/2015/01/14/360134/europe-amazed-by-gazproms-intention-to-redirect-all-gas-from-ukraine-to-turkey.html
http://www.euractiv.com/sections/energy/russia-says-it-will-shift-gas-transit-ukraine-turkey-311291
https://euobserver.com/news/127216
http://rt.com/business/222619-bulgaria-south-stream-gazprom/