Elsha of the Infinite

• WOTC Official Reminder Textofficial rules
  Prowess — Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.

• WWizards of the Coast2019-08-23
  Official Ruling
  You'll still pay all costs for a spell you cast from your library, including additional costs. You may also pay alternative costs.
• WWizards of the Coast2019-08-23
  Official Ruling
  Elsha lets you look at the top card of your library whenever you want (with one restriction—see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.
• WWizards of the Coast2019-08-23
  Official Ruling
  If the top card of your library changes while you're casting a spell, playing a land, or activating an ability, you can't look at the new top card until you finish doing so. This means that if you cast the top card of your library, you can't look at the next one until you're done paying for that spell.

Rulings via Scryfall