Rule 702.188a

Web-slinging is a static ability that functions while the spell with web-slinging is on the stack. “Web-slinging [cost]” means “You may cast this spell by paying [cost] and returning a tapped creature you control to its owner’s hand rather than paying its mana cost.” Casting a spell using its web-slinging ability follows the rules for paying alternative costs in rules 601.2b and 601.2f–h.