if i am correct An-Nahyi’l Akīd is a book explaining the reason as to why salah is rejected behind the ghayr muqallideen. it is in vol. 6 of fatawah ar-ridawiyyah shareef.
mashaAllah may Allah bless the Great Shaykh immensely and enable them to carry on their immense service to the Ahlus Sunnah
I just received some good news from a brother in Madinah Al Munawwarah that Sayyidi Taj al-Shari'ah, Mufti Akhtar Rida Khan has completed the translation of Imam Ahmad Rida Khan's An-Nahyi’l Akīd ani’s Salati Wara’a Adda’t Taqlid into Arabic. It was amazing news to me as I requested him to translate the work into Arabic when he blessed us here in South Africa with his presence. He agreed to it and now Al-Hamdulillah he fulfilled the humble request of a poor Sunni.