Sometimes transfer rumours come through the deep, dark places of the internet (e.g. the Daily Mail). Seedy and untrustworthy, chasing them can often lead to false hope and end with a cynical rejection of all things whispered.
Sometimes, however they come straight from the clubs themselves:
That Borussia Monchengladbach want to get Thorgan Hazard permanently is of course no secret at all. The Belgian has been effective in the Bundesliga since moving from Zulte Weregem in the summer, and at 21 he'll only get better. But it wouldn't be surprising if Chelsea vetoed this winter-- if he keeps up his form for an extended period, his value will be much higher than it is right now.
But, as the above tweet demonstrates, saying no in the winter won't stop Gladbach from pressing the issue until it's resolved one way or the other.