Most every class that can cast a spell has a Spell Save DC, which is typically
DC = 8 + proficiency + [ability modifier]
However, if a monk follows the Way of the Four Elements, they gain access to several abilities that either mimic existing spells with saving throws (Burning Hands) or create new spell-like abilities with saving throws (Fist of Unbroken Air).
However, these spells don't explicitly spell out which ability modifier should be used for the save DC. I have gone with Wisdom in personal games, as it's something that Monks already have a good deal of, but I cannot find anything in the PHB backing this up.
Which ability, according to the rules, should Monks use to get their spell save DC?