Naruto Discussion about Hashirama's sage mode


in [this answer to "Is there a slug based Sage
Mode?"](https://anime.stackexchange.com/a/11746/1587) it says

> He learnt Sage Mode in Shikkotsu Woods/Forest, a legendary place that is
> equally famous as Mount Myōboku (toad) and Ryūchi Cave (snake).

I was wondering if someone could direct me to the manga panels/anime episodes
where it was mentioned that Hashirama trained at Shikkotsu Forest?

