Historical Rules · 5 October 2018
Rule 508.3a
Official source text from the 5 October 2018 Rules release
An ability that reads “Whenever [a creature] attacks, . . .” triggers if that creature is declared as an attacker. Similarly, “Whenever [a creature] attacks [a player or planeswalker], . . .” triggers if that creature is declared as an attacker attacking that player or planeswalker. Such abilities won’t trigger if a creature is put onto the battlefield attacking.