Urza's Armor — ruling 1

If a spell or ability damages multiple things, divide up the damage before applying this effect, and prevent the 1 damage to you only.

Urza's Armor — ruling 2

It is a prevention effect applied when damage would be dealt.

Urza's Armor : règles, fonctionnement et interactions (MTG)

Urza's Armor 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

If a source would deal damage to you, prevent 1 of that damage.

How it works : Urza's Armor

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
If a spell or ability damages multiple things, divide up the damage before applying this effect, and prevent the 1 damage to you only.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
It is a prevention effect applied when damage would be dealt.
No other card is named in this ruling.