What is the amount of impurity that necessitates ritual purification, especially when a small amount of urine remains inside the urethra after urination, and what is the ruling on this amount touching clothes after movement and walking?

The obsessive doubt (waswasah) about urine exiting after complete ablution (istinja') is unfounded. The default assumption is that nothing has exited unless there is certainty. The viscous substance inside the glans penis does not necessitate ghusl (ritual bath), nor does the sticking together of the sides of the glans penis. Inspecting the penis is an unlawful innovation (bid'ah). It is advised to ignore these obsessive thoughts and to sprinkle a small amount of water on the penis to put an end to them.