What is the ruling on a congregant praying behind an imam if the congregant cannot discern whether the imam pronounces the letter "ṭa" (ط) or "ta" (ت) in His (the Most High's) saying: "the Straight Path" (الصراط المستقيم)?

If there are [Satanic] whispers (waswasah), they must be ignored and struggled against. The default assumption regarding the Imam's recitation is that it is correct, and its invalidity should not be determined except with certainty. Many scholars have been lenient concerning letters with close points of articulation, such as the letters Taa' (ت) and Taa' (ط), and they did not invalidate the prayer of common people due to errors in them, this being to ease matters for people and due to the widespread nature of the affliction.