Sahih Muslim 874
صحيح مسلم#874SahihUbada b. as-Samit reported from the Messenger of Allah (may peace be upon him ):He who does not recite Fatihat al-Kitab is not credited with having observed the prayer
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 4:37