Historical Rules · 13 October 2023
Rule 107.3n
Official source text from the 13 October 2023 Rules release
If a delayed triggered ability created by a resolving spell or ability refers to X, X is not defined in the text of that triggered ability, and the spell or ability that created it had a value of X chosen for any of its costs, the value of X for the triggered ability is the same as the value of X for the spell of ability that created it.