Does doubt affect the validity of prayer, and is there a difference of opinion regarding this?

Doubt in prayer is divided into two types:

The first type: Doubt regarding the commencement of prayer (the intention or the opening takbeer): This type necessitates repeating the prayer from the beginning, because doubt is considered as non-action. An exception to this is one who frequently experiences doubt and waswasah (whispers from Satan), who does not need to repeat.

The second type: Doubt regarding the obligatory acts of prayer after the opening takbeer: This type does not invalidate the prayer. Rather, one acts based on what is most probable. Otherwise, one should build upon certainty (the lesser number), then perform the prostrations of forgetfulness (sujood as-sahw) before the tasleem.

The Messenger of Allah, peace and blessings be upon him, said: "If one of you doubts in his prayer and does not know how many he has prayed—three or four—then he should cast away doubt and build upon what he is certain of, then prostrate twice before giving the tasleem."