Fist of Suns

• WWizards of the Coast2004-12-01
  Official Ruling
  If you apply Fist of Suns's alternative cost to a spell with in its mana cost, X is 0.
• WWizards of the Coast2004-12-01
  Official Ruling
  Fist of Suns's ability is an alternative cost to cast a spell. You can't combine this with other alternative costs, such as flashback. You can pay additional costs, such as kicker, in addition to this alternative cost.
• WWizards of the Coast2004-12-01
  Official Ruling
  If you cast a spell for which mana of any color can be spent to cast it, you may cast it for Fist of Suns's alternative cost and still spend any color of mana to cover that cost.
• WWizards of the Coast2014-02-01
  Official Ruling
  If you pay rather than pay the mana cost of a spell with in its mana cost, the only legal choice for X is 0.

Rulings via Scryfall