Rule 428.5.d

Abilities originating from Game Objects that are attributed Kill Actions are attributed in addition to the Game Object that created them. Example: There is a spell that says “Do this twice: Deal 3 to a unit.” Immortal Phoenix is a unit that says “When you kill a unit with a spell, you may pay [1][C] to play me from your trash.” A player plays the spell while Immortal Phoenix is in their trash. The “do this” phrasing means that it has a reflexive triggered ability, which places two triggered abilities on the chain. As each of those triggered abilities resolve, it deals damage to the unit chosen for that ability. If one of these abilities deals lethal damage to a unit, both the spell and its ability are considered sources of the damage, and so both the spell and its ability receive attribution for killing the unit. This means that the spell’s controller killed a unit with a spell, so Immortal Phoenix’s ability will trigger.