Using toilet paper for istinjah
In the name of Allah, the most Beneficent, the most Merciful.
The act of purifying oneself of the impurities excreted via the hind and front private parts is called istinjah. It is an emphasised sunnah to do istinjah until the place becomes clean and pure. If the impurities spread beyond the area of the outlet by more than the size of a dirham it will be wajib to use water. (Raddul Muhtar p.550 v.1)
With regards to your question, after urinating if the impurity did not spread by more than the size of a dirham it will be permissible for you to suffice on tissue paper. (Nuurul Idaah p.27)
If there is a sign or mark of the impurity on your underwear it will be necessary to wash it off before commencing the salah. The urine of human beings is considered as a Najaasat Ghaleeza (heavy type of impurity) and washing is wajib if the impurity covers an area greater than the size of a dirham. If it is less than a dirham, then the salah performed in this manner will be discharged. This does not mean one should leave minor impurities on one’s clothes, as to perform salah without washing even the minimum allowed limit is makruh. (Nurul Idaah p.51 –p.54 & p.88)
The conclusion to your question is that if there are traces of impurities on your underwear then you should wash it off before commencing the salah. If you wish, you may change your underwear for a clean pair to remove all doubt from your mind before performing salah.
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham