Hummels refusing to comment on United talk
The Germany international has long been regarded as one of the Premier League's club top targets and previously indicated that he would be open to leaving Dortmund last month.
Speculation over his future continues to persist, but when pressed on the topic by Bild, Hummels said: "I don't want to talk about that topic constantly.
"I did it a few weeks ago - and that must be enough."
Asked about reports of a bid from United, Dortmund sporting director Michael Zorc replied: "I can't confirm this."
Hummels has made 26 appearances this season for a Dortmund side that sits ninth in the Bundesliga having spent much of the campaign in the lower reaches of the table.