Narrated 'Ubada bin As-Samit:
Allah's Messenger (ﷺ) said, "Whoever does not recite Al-Fatiha in his prayer, his prayer is invalid."
Abu Huraira reported: 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).
Abu Huraira reported: The Messenger of Allah (ﷺ) said: If anyone observes prayer in which he does not recite Umm al-Qur'an, It is deficient [he said this three times] and not complete. It was said to Abu Huraira: At times we are behind the Imam. He said: Recite it inwardly, for he had heard the Messenger of Allah (ﷺ) declare that Allah the Exalted had said: I have divided the prayer into two halves between Me and My servant, and My servant will receive what he asks. When the servant says: Praise be to Allah, the Lord of the universe, Allah the Most High says: My servant has praised Me. And when he (the servant) says: The Most Compassionate, the Merciful, Allah the Most High says: My servant has lauded Me. And when he (the servant) says: Master of the Day of judg- ment, He remarks: My servant has glorified Me. and sometimes He would say: My servant entrusted (his affairs) to Me. And when he (the worshipper) says: Thee do we worship and of Thee do we ask help, He (Allah) says: This is between Me and My servant, and My servant will receive what he asks for. Then, when he (the worshipper) says: Guide us to the straight path, the path of those to whom Thou hast been Gracious not of those who have incurred Thy displeasure, nor of those who have gone astray, He (Allah) says: This is for My servant, and My servant will receive what he asks for. Sufyan said: 'Ala b. 'Abd al-Rahman b. Ya'qub narrated it to me when I went to him and he was confined to his home on account of illness, and I asked him about it.