Historical Rules · 7 August 2020
Rule 107.3k
Official source text from the 7 August 2020 Rules release
If an object’s enters-the-battlefield triggered ability or replacement effect refers to X, and the spell that became that object as it resolved had a value of X chosen for any of its costs, the value of X for that ability is the same as the value of X for that spell, although the value of X for that permanent is 0. This is an exception to rule 107.3h.