Assalaamu alaykum. When washing my impure clothes, there was one incident where one of my undergarments had stains remaining while other clothes were clean from impurity. So can I wear that undergarment, or do I have to wash it and also clean the other clothes (all clothes have been washed three times in the washing machine)?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

You are permitted to wear these clothes, but if you are certain that these remaining stains are from impurity, then you are obliged to wash them if you wish to pray in them.

However, if these stains are just a color; then the traces of the color and the effect of impurity after washing do not harm if it is hard to remove them, as the Prophet said to Khawlah “It is sufficient to use water, and its effect [i.e. the remaining color of impurity] does not harm." [Abu Dawood - Al-Albaani graded it saheeh (sound)]

For more benefit, please refer to fatwa 7842.

As regards other clothes, you are not obliged to wash them, even if the piece in which spots remained still has impurity on it, unless its impurity changed the water, which is highly unlikely, especially given that you mentioned that they have been washed three times. For more benefit, please refer to fatwa 229372.

Allaah knows best.