I heard the Messenger of Allah ﷺ say, "Any prayer in which the Opening of the Book is not recited is deficient."
حدثنا الفضل بن يعقوب الجزري حدثنا عبد الأعلى عن محمد بن إسحق عن يحيى بن عباد بن عبد الله بن الزبير عن أبيه
سمعت رسول الله ﷺ يقول كل صلاة لا يقرأ فيها بأم الكتاب فهي خداج
[Its isnad is hasan] Ahmad reported it, 6/275, from Ibn Ishaq, who said, "Yahya bin Abbad narrated it to me," with the rest of the text, with a slight difference. The hadith has many corroborating narrations, see the following hadith.