Sahih Muslim Book 4, Hadith Number 775.
Abu Huraira reported: The Apostle of Allah (may peace be upon him) 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 (may peace be upon him) 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: 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.