Board Thread:Rebirth: Applications/@comment-1875854-20141110174716/@comment-1875854-20141127014538

It's in the explanation I gave you. He (Hashirama) was able to learn normal Sage Mode at first. His wood release originally brought him to the idea of it, but he doesn't need Wood Release to enter Sage Mode. That was to show how Hashirama studied from his own techniques to enter Sage Mode.

And then he learned how to enter Sage Mode like say Kabuto, and it was this he taught to Kachuu.