You must Guard Yourself from the Impurity of Urine
Question :
Sometimes after praying, I discover that there are some drops of urine or (other) impurity on my clothes, and sometimes I do not discover it until the following day; is my prayer then correct, and is there anything incumbent upon me?
Answer:
You must guard yourself against the impurity of urine and the like; do not begin Istinja' and ablution until the flow of urine has completely ceased. If you are afflicted with dribbling or something similar to continuing urine, then you should urinate an hour or so before the time of prayer and wait for it to cease, then make ablution. But if you fear the whispering (of Satan), then sprinkle water on your garments and your undergarments, so that Satan may not say to you that this wetness is caused by urine. If the urine is continual and unceasing, then it is permissible to pray with it, but do not make ablution until after the Adhan. If it stops, and you observe wetness and you are sure that it occurred before prayer, then it is better to repeat that prayer; but if you are in doubt about it, then you do not have to repeat it, Allah, the Most High willing.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Page 37