underwear

Dear Scholar, salamu alaikum I have a problem with having madhi on underwear. Some times it's not under my control. I do not know if it is necessary to wash my underwear. Some times I am not at home and it cause me to wonder how can I pray, and I miss the prayer. Can I pray with out purifying my clothes when I am not at home? And how can I purify my clothes? should I wash it or just wet it  three times?thank you very much. wassalamu alaikum