The Messenger of Allah (ﷺ) said: He who said his prayer, but did not recite the opening chapter of al-Kitab, his prayer is incomplete. He repeated it thrice.
Sahih Muslim Book 4, Hadith 43
← Previous Hadith He who observed prayer but he did not recite the Umm al-Qur'an in it, and the rest of the hadith is the same as transmitted by Sufyan, and in this hadith the words are:" Allah the Most High said: the prayer is divided into two halves between Me and My servant. The half of it is for Me and the half of it is for My servant."

Sahih Muslim Book 4, Hadith 42 Next Hadith → The Messenger of Allah (ﷺ) said: One is not credited with having observed the prayer without the recitation (of al-Fatiha). So said Abu Huraira: (The prayer in which) the Messenger of Allah (ﷺ) recited in a loud voice, we also recited that loudly for you (and the prayer in which) he recited inwardly we also recited inwardly for you (to give you a practical example of the prayer of the Holy Prophet).

Sahih Muslim Book 4, Hadith 44