If washing powder has any Najis substances in it, is it permissible to pray with the clothes which were washed from that powder in the washing machine?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
In most cases, the animal by-products found in detergents and washing powders go through a complete transformation. Hence, such products will be considered pure and it will be permissible to use them to wash one's clothes. Therefore, praying in such clothes will be allowed.
Only Allah knows best
Written by Maulana Belaal Ahmed
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 AL Bahrur-raiq Vol 1 pg 239 (Darul Kutub al Ilmiya print)