Historical Rules · 5 February 2021
Rule 116.2g
Official source text from the 5 February 2021 Rules release
A player who has chosen a companion may pay {3} to put that card from outside the game into their hand. This is a special action. A player can take this action any time they have priority and the stack is empty during a main phase of their turn, but only if they haven’t done so yet this game. (See rule 702.138, “Companion.”)