What is the ruling on someone who touched his anus due to itching through his clothes, assuming his clothes were clean, then found a yellow stain (feces residue) on his underwear the next day? Is he required to repeat the prayers he performed while in that state? And what if he is unsure whether this occurred before or after the 'Isha prayer?

If you ascertain that some excrement exited when you felt an itch, and you found a trace of it after several prayers, then scholars have differed on the obligation to repeat the prayers, and the more cautious approach is to repeat them. However, if you are unsure whether anything exited, you are not required to repeat [the prayers], because the default assumption is that nothing exited unless there is certainty. And if you doubt whether the excrement exited before or after the prayer, then consider that it exited after it, and your prayer is valid, because the default principle is that an impurity is attributed to the nearest of its possible times.