I'm looking for a specific episode of Naruto in the war arc where Tobirama makes a comment about Naruto and him learning Sage Mode at Mount Myōboku. If I remember it was some time when they learned that Sage Chakra is effective at combating Madara.
There are two instances where Tobirama compliments Naruto.
The first instance is when they learn that only Senjutsu works on The Ten-Tails' Jinchūriki: Obito. In Episode 379, they discover about it and in episode 380, Tobirama compliments Naruto because he feels he is fighting beside his elder brother Hashirama.
In Episode 381, he compliments Naruto as he thinks he might be the only one to surpass Hashirama as the Hokage.