Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

If you did see a brown stain on your underwear, then you should repeat your prayer. It is an indication that you broke your wudu. If you did not find any such stain, then there is no need to repeat the prayer. You should dispel such doubts as due to the whisperings of Satan.

 

Allah Almighty knows best.