He who does not recite Fatihat al-Kitab is not credited with having observed the prayer.
Sahih Muslim Book 4, Hadith 37
← Previous Hadith I and 'Imran b. Husain said prayer behind 'Ali b. Abu, Talib. He recited takbir when he prostrated, and he recited takbir when he raised his head and he recited takbir while rising up (from the sitting position at the end of two rak'ahs). When we had finished our prayer, 'Imran caught hold of my hand and said: He (Hadrat Ali) has led prayer like Muhammad (ﷺ) or he said: He in fact recalled to my mind the prayer of Muhammad (may peace be upon him.)

Sahih Muslim Book 4, Hadith 36 Next Hadith → The Messenger of Allah (ﷺ) said: He who does not recite Umm al-Qur'an is not credited with having observed the prayer.

Sahih Muslim Book 4, Hadith 38