A lot of Muslims wear trousers such as jeans that are not necessarily baggy. However when they go into ruku’ or sujood whilst in prayer, their lower back tends to show. Is this person’s Salaah valid? What about if his lower back shows his underwear instead of his actual skin?
The correct opinion is that Isbaal is haram and not allowed. Dr. Saleh As-Saleh has done an excellent discussion on the topic in his book on Isbaal found here.
Faisal ibn Abdul Qaadir Ibn Hassan