I trust that this correspondence reaches you in the best of health and iman:
To continue:
Quranic Verse Concerning Whisperings
Allah provides us with a simple and very effective way of dealing with whisperings of the devil in the following verse:
وَإِمَّا يَنزَغَنَّكَ مِنَ الشَّيْطَانِ نَزْغٌ فَاسْتَعِذْ بِاللّهِ إِنَّهُ سَمِيعٌ عَلِيمٌ Qur’an 7:200
“And if a false imputation from the Shaytan afflicts you, seek refuge in Allah; surely He is Hearing, Knowing.”
Al-Raghib and Ibn Atiyyah have restricted the word “nazghun” to apply to inserting one thing into another in order to corrupt it. The devil thus seeks to infuse you with doubts and misgivings about the validity of your prayer, attempting to corrupt your prayer and acts of worship with his insinuations. These insinuations are nothing but baseless doubts which do not affect the certainty of your condition, and thus these doubts should be dealt with clinically and removed from your mind and heart. These doubts appear like looming darkness, but they are quick to dissipate when you apply the rule found below.
Principle:
اليقين لا يزول بالشك
Certainty is not removed or dispensed of based on doubt.
To return to the verse quoted above, the devil attempted this with our beloved master Muhammad, and he was commanded to seek refuge in Allah. I heard Shaykh Sha’rawi mention that whenever you are beset with these type of imputations from the devil, even during the course of your prayer, you should say:
أعوذ بالله من الشيطان الرجيم
The only addition that I could advise in the initial stages of resolving these imputations from the devil would be to use other forms of the isti’adhah. Here is one such form that you may use in addition to the one cited above:
أللهم إني أعوذبك من الشيطان الرجيم من همزه ونفخه ونفثه
In conclusion, unless you are absolutely certain that it is urine particles that you can clearly see (since urine particles that cannot be seen with the naked eye are overlooked) or flatulence (that you can smell and discernably experience its emission), you simply need to seek refuge in Allah, dispel these misgivings, and carry on.
May Allah enable you to conquer this insidious enemy and allow you to bask in the light of certainty and resolve.
All the very best.
PS: The above has been empirically proven to work.