Answered by Shaykh Yusuf Weltch
If underwear becomes slightly wet from wadhi or anything, must the trousers be changed too? And if I’m uncertain if I touched the wet spot on my underwear but I likely did? I continued touching things as I wasn’t paying attention.
If soap splashes on my body while washing my hands, must the splashes be washed?
If najasa has gotten onto my inner thigh and I spray it with a bidet shower and the water splashes everywhere due to the pressure, can I assume I am clean or not?
In the Name of Allah, the Most Merciful and Compassionate
Excused Amount of Impurity
You do not need to change your trousers if there is a small amount of impurity. Impurity is excused as long as it does not exceed 5gms in weight or 5cm in diameter. [Shurunbulali, Maraqi al -Falah]
Note that 5gms and 5cms is a lot and highly unlikely even if one does not perform istinjaa’ at all.
Impurity does not transfer merely by touching something moist and impure. [Shurunbulali, Maraqi al-Falah]
If the garment you touch is dripping wet and traces of impurity (like smell or color) are present on the hand after touching, the hand is impure. [Shurunbulali, Maraqi al-Falah]
Reasonable Surety and Not Second Guessing
When washing the impure hand or while washing the impurity on your leg, any splashing should be pure unless you can see traces of impurity.
Note it is not an obligation to investigate the presence of impurity. Assume everything is pure till reasonably you are sure otherwise, after making a decision of purity, never second guess your decision.
We pray to Allah Most High to free your mind from these misgivings so you can focus on Him.
Hope this helps
And Allah knows best
[Shaykh] Yusuf Weltch
Checked and Approved by Shaykh Faraz Rabbani