The Messenger of Allah (ﷺ) said: If anyone observes prayer in which he does not recite Umm al-Qur’an (i.e., Al-Fātiḥah), it is deficient [he said this three times] and incomplete. It was said to Abu Hurayraa: 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 judgment, 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: You do we worship and of You 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 along the straight path, the path of those to whom You have been Gracious, not of those who have incurred Your 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. Reference: Sahih Muslim 395