The Prophet ﷺ said, "Any prayer in which the Opening of the Book is not recited is deficient, deficient, deficient, incomplete."
حدثنا محمد بن جعفر قال حدثنا شعبة قال سمعت العلاء يحدث عن أبيه
عن النبي ﷺ أنه قال كل صلاة لا يقرأ فيها بأم الكتاب فهي خداج فهي خداج فهي خداج غير تمام
Its isnad is sahih. It was previously mentioned at 6903, and it is found in Muslim 1/297 no. 395, in the chapter on prayer, on the obligation of reciting Fatihah, and in Abu Dawud 1/216 no. 821, in the chapter on prayer, on one who omits recitation in his prayer, and in Nasai 2/135 no. 909, and in Malik 1/84.