What is the ruling on one who experiences a whisper of polytheism during prayer, telling him to intend the worship of [his] whims instead of Allah?

Satan wants to distance you from the worship of your Lord through whispers (waswas), so you must strive against him and maintain your prayers, and pay no attention to these whispers. Indeed, disregarding them is one of the most effective means of treatment. This waswas does not affect the validity of your prayer. Increase in the remembrance of Allah, the recitation of the Quran, and supplication to Him so that He may remove what afflicts you.