Historical Rules · 22 July 2016
Rule 702.92a
Official source text from the 22 July 2016 Rules release
Undying is a triggered ability. “Undying” means “When this permanent is put into a graveyard from the battlefield, if it had no +1/+1 counters on it, return it to the battlefield under its owner’s control with a +1/+1 counter on it.”