In his youth, Bartok had several sources. At first, Brahms, because
his professor of composition was a great Brahms admirer. Next, Bartok
was very enthusiast when he discovered Zarathustra by Richard Strauss.

However, later he moved away from these sources.

His permanent sources were:

* The Hungarian popular (peasant) music
* Debussy
* Bach
* Beethoven

If you want details about some of these source, please ask, and I'll
try to answer.

Best, Pierre