Choosing sources must include delayed triggers

E.g. _Searing Blood_

> 609.7a. If an effect requires a player to choose a source of damage, they may choose a permanent; a spell on the stack (including a permanent spell); any object referred to by an object on the stack, by a replacement or prevention effect that's waiting to apply, or by **a delayed triggered ability that's waiting to trigger** (even if that object is no longer in the zone it used to be in)