Did Buddha ever deny the existence of God? I heard that when Buddha was asked about God, he kept silent. 

Although perhaps in Buddhism nothing really exists, did Buddha ever deny the existence of God?

Hasn't he really never spoken about God?
Here: Tittha Sutta: Sectarians


  Monks, there are these three sectarian guilds that — when cross-examined, pressed for reasons, &amp; rebuked by wise people — even though they may explain otherwise, remain stuck in [a doctrine of] inaction. Which three?
  
  "There are brahmans &amp; contemplatives who hold this teaching, hold this view: 'Whatever a person experiences — pleasant, painful, or neither pleasant nor painful — that is all caused by what was done in the past.' There are brahmans &amp; contemplatives who hold this teaching, hold this view: 'Whatever a person experiences — pleasant, painful, or neither pleasant nor painful — that is all caused by a supreme being's act of creation.' There are brahmans &amp; contemplatives who hold this teaching, hold this view: 'Whatever a person experiences — pleasant, painful, or neither pleasant nor painful — that is all without cause &amp; without condition.




Also here: A Discourse to Knowers of Veda -- Tevijja Sutta


  “And just even so, VāseMMha, though you say that the Brahmans are not able to point out the way to union with that which they have seen, and you further say that neither any one of them, nor of their pupils, nor of their predecessors even to the seventh generation has ever seen Brahmā. And you further say that even the Rishis of old, whose words they hold in such deep respect, did not pretend to know, or to have seen where, or whence, or whither Brahmā is. Yet
  these Brahmans versed in the Three Vedas say, forsooth that they can point out the way to union with that which they know not, neither have seen! Now, what think you, VāseMMha? Does it not follow that, this being so, the talk of the Brahmans, versed though they be in the Three Vedas, is foolish talk?”
  
  VāseMMha, is there a single one of the Brahmans versed in the three Vedas who
  has ever seen Brahmā face to face?”