Toshiro Umezawa — ruling 1

Creatures destroyed by an instant that you cast normally can cause a trigger that allows the instant to be cast again. For example, cast Terror on an opponent’s creature. The creature is destroyed, triggering Toshiro’s ability, and then the Terror is put into your graveyard. You can put Toshiro’s ability on the stack targeting the Terror and then recast it when the ability resolves.

Toshiro Umezawa — ruling 2

You must still pay the mana cost of the target instant in your graveyard to cast it using Toshiro’s ability.

Toshiro Umezawa : règles, fonctionnement et interactions (MTG)

Toshiro Umezawa is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Bushido 1 (Whenever this creature blocks or becomes blocked, it gets +1/+1 until end of turn.) Whenever a creature an opponent controls dies, you may cast target instant card from your graveyard. If that spell would be put into a graveyard, exile it instead.

How it works : Toshiro Umezawa

Original English text — Wizards of the Coast / Scryfall

#1January 1, 2011· Wizards of the Coast
Creatures destroyed by an instant that you cast normally can cause a trigger that allows the instant to be cast again. For example, cast Terror on an opponent’s creature. The creature is destroyed, triggering Toshiro’s ability, and then the Terror is put into your graveyard. You can put Toshiro’s ability on the stack targeting the Terror and then recast it when the ability resolves.
No other card is named in this ruling.
#2February 1, 2005· Wizards of the Coast
You must still pay the mana cost of the target instant in your graveyard to cast it using Toshiro’s ability.
No other card is named in this ruling.