Historical Rules · 3 May 2019
Rule 702.48a
Official source text from the 3 May 2019 Rules release
Ninjutsu is an activated ability that functions only while the card with ninjutsu is in a player’s hand. “Ninjutsu [cost]” means “[Cost], Reveal this card from your hand, Return an unblocked attacking creature you control to its owner’s hand: Put this card onto the battlefield from your hand tapped and attacking.”