Relentless Skaabs

• WOTC Official Reminder Textofficial rules
  Undying — When this creature dies, 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.

• WWizards of the Coast2011-01-22
  Official Ruling
  When Relentless Skaabs returns to the battlefield because of its undying ability, it’s not being cast. You won’t exile a creature card from your graveyard.
• WWizards of the Coast2013-04-15
  Official Ruling
  You must exile exactly one creature card from your graveyard to cast this spell; you cannot cast it without exiling a creature card, and you cannot exile additional creature cards.
• WWizards of the Coast2013-04-15
  Official Ruling
  Players can only respond once this spell has been cast and all its costs have been paid. No one can try to otherwise remove the creature card you exiled in order to prevent you from casting this spell.

Rulings via Scryfall