Do I have to change my underwear for every prayer? May Allaah reward you with good.
If you are certain that some urine has come out, then you have to repeat your wudoo’ and wash off what has got onto your clothes. If the site of the urine is not clear to you, then you should wash the parts where you think it most likely got onto, so that you can be certain that the impurity has been removed.
It says in Zaad al-Mustaqni’: If the location of the impurity is not clear, it should be washed until one is certain that it has been removed.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh: meaning if the impurity got onto something, and its location is unclear, then it is essential to wash whatever it got onto so as to be certain that it has been removed. It should be noted that the area the impurity gets onto will be one of two things: either it will be small or it will be big. If it is big, then he should do his best to wash whatever he thinks most likely the impurity got onto, because washing all of the large area is difficult. And if it is small, then he has to wash it so that he can be certain that it has been removed. End quote from al-Sharh al-Mumti’ (1/435).
Thus you will know that a small amount of urine is not excused.
You should beware of waswasah, because it is a disease and an evil; if it gains power over a person it will cause him distress, instil doubt in him and make worship difficult for him. So beware of that.
Hence we say: Do not inspect your clothes and do not worry about this matter until it leaves you.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: When I finish my wudoo’ and go to pray, I feel that a drop of urine has come out on my private parts. What do I have to do?
He replied: What he should do is ignore this, as enjoined by the imams of the Muslims, and not pay any attention to it or look at his private parts to see whether anything has come out or not. If he seeks refuge with Allaah from the accursed shaytaan and ignores it, it will disappear. But if he is certain, absolutely certain, then it is essential to wash off whatever urine has got on him and repeat his wudoo’, because some people, if they feel some coolness on the head of the penis, think that something has come out. But if you are certain, then it is as I have said to you. What you are saying is not incontinence, because it stops and starts, whereas incontinence is continual. But this (your case) means that one or two drops come out when you move. This is not incontinence, because once a couple of drops have come out, it stops. In this case you should wash it and do wudoo’ again, and do this all the time. Be patient and seek reward with Allaah. End quote from Liqa’ al-Baab al-Maftooh (15/184).
As for urinary incontinence, that means that it comes out without stopping at any particular time. The one who has this problem has to wash his private part and wrap it so as to prevent the urine from getting onto his body or clothes, and do wudoo’ after the time for prayer has begun.
Please see also the answer to question number 39494 for information on the rulings on urinary incontinence.
And Allaah knows best.