It doesn't exist, never did. It's a heavy misconception. Someone misread what the databook actually said, and then people have taken it as fact for a long time, without ever.
It's now become a popular belief over the internet, whlie in fact it is just false information.
Databook 1:
He was a superior ninjutsu user, being able to use Water Release without being near the waterside, among many other feats.
(something that every water user can do by Shippuden ie Kisame, Kakashi, Itachi, Kabuto, Mei, Hiruzen... everyone and their mothers can do)
Databook 2:
Shodai's younger brother, who greatly contributed to Konoha's establishment as a
system. In contrast to his calm older brother, his nature is to push forward
towards his objective with passion and enthusiasm. He is prideful of his
superior combat ability, and with the Suiton jutsu which he was especially
skilful at he was unexpectedly revived in the present to defeat Sandaime, which
put fear in the people of Konoha.
^^^
Somehow, this translated to Tobirama = best water style user ever
