Ali from Cairo — ruling 1

This effect does not apply to effects which reduce your life without doing damage.

Ali from Cairo — ruling 2

The ability works up until Ali enters the graveyard, so if he takes lethal damage or is destroyed at the same time you take damage, the ability helps you. If the damage occurs after it goes to the graveyard, however, it is not affected by the Ali which is no longer on the battlefield.

Ali from Cairo — ruling 3

This effect does not prevent damage, it prevents the damage from turning into loss of life. So the full damage is dealt (and abilities that trigger on damage being dealt still trigger), but the full loss of life is not applied.

Ali from Cairo : règles, fonctionnement et interactions (MTG)

Ali from Cairo 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

Damage that would reduce your life total to less than 1 reduces it to 1 instead.

How it works : Ali from Cairo

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
This effect does not apply to effects which reduce your life without doing damage.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
The ability works up until Ali enters the graveyard, so if he takes lethal damage or is destroyed at the same time you take damage, the ability helps you. If the damage occurs after it goes to the graveyard, however, it is not affected by the Ali which is no longer on the battlefield.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
This effect does not prevent damage, it prevents the damage from turning into loss of life. So the full damage is dealt (and abilities that trigger on damage being dealt still trigger), but the full loss of life is not applied.
No other card is named in this ruling.