Board Thread:Theories and Speculation/@comment-3001903-20161001075236/@comment-28323245-20161001103630

Tobirama has seen/studied a lot the Uchiha clan and probably also the sharingan so he might be used to or has seen Amaterasu before those saying that Sasuke's Kagutsuchi is one of the strongest.

I think that you are talking about this Scorch Release: Halo Hurricane Jet Black Arrow Style Zero

Tobirama doesn't refere to Kagutsuchi has an individual jutsu, but more to the combined rasenshuriken and kagutsuchi or a "Cooperation Ninjutsu"