Here we go a quick search reveals a 1977 interview http://www.rollingstone.com/movies/news/the-wizard-of-star-wars-20120504

Ben almost kills Vader. As a matter of fact, he falls into a volcanic pit and gets fried and is one destroyed being

Also the ROTJ novelization

And this memory brought a wave of other memories with it. Memories of brotherhood, and home. His dear wife. The freedom of deep space. Obi-Wan.
Obi-Wan, his friend … and how that friendship had turned. Turned, he knew not how - but got injected, nonetheless, with some uncaring virulence that festered, until … hold. These were memories he wanted none of, not now. Memories of molten lava, crawling up his back … no.
This boy had pulled him from that pit - here, now, with this act. This boy was good.