Sphinx of Foresight has the following effect:
You may reveal this card from your opening hand. If you do, scry 3 at the beginning of your first upkeep.
Imagine the following Scenario:
- Player A plays first, Player B draws first.
- Player B reveals Sphinx of Foresight.
- Player A plays a Swamp and casts Thoughtseize targeting the Sphinx of Foresight.
- Player A ends his turn.
- Player B gets to his upkeep. Is he allowed to scry now?
I couldn't find any remarks regarding this topic in the gatherer.