What is the ruling on wearing torn underwear, knowing that trousers are worn over it?

If the trousers cover the 'awrah (private parts) that might be exposed by a hole in the underwear, then there is no harm in wearing them. However, if they do not cover it, then it is obligatory to cover it with any concealing garment during prayer or in the presence of people. This is based on the statement of Ibn Qudamah: "It is obligatory to cover the 'awrah with something that conceals the color of the skin, whether it be clothes, leather, or other materials. If it describes the color of the skin, it is not considered, because it is not a covering." Ibn 'Uthaymeen said: "All pure, permissible clothes, if they cover the 'awrah, then it is permissible to pray in them, even if they are sleepwear. There is no requirement for specific clothes, but the clothes must be pure, permissible, and concealing. If these conditions are met, then there is no harm in praying in them."