On the authority of Abu Huraira (may Allah be pleased with him) from the Prophet (peace be upon him), who said:
The prayer of someone who has not recited the Essence of the Quran (1) during it is deficient (and he repeated the word three times), incomplete. Someone said to Abu Hurayrah: [Even though] we are behind the imam? (2) He said: Recite it to yourself, for I heard the Prophet (peace and blessings of Allah be upon him) say: Allah (mighty and sublime be He), had said: I have divided prayer between me and my servant into two halves, and My servant will have what he asked. When the servant says: Al-hamdu lillahi rabbi l-alamin (3), Allah (mighty and sublime be He) says: My servant has praised Me. And when he says:-Rahmani r-Rahim (4), Allah (mighty and sublime be He) says: My servant praised me, and when he says: Maliki yawmi d-din (5), Allah says: My servant glorified Me - and on one occasion He said: my servant has submitted to my power. And when he says: Iyyaka in budu wa iyyaka nasta in (6), he says: This is between Me and My servant, and My servant shall have what he asked. And when he says: Sirata Ihdina s-l-Mustaqim, siratal ladhina one amta alayhim ghayril-maghdubi alayhim wa la d-dallin (7), He says: This is for My servant, and My servant will have what you ordered.
(1) Surat al-Fatiha, the first sura (chapter) of the Qur'an.
(2) ie standing behind the imam (leader) listening to him reciting al-Fatihah.
(3) "Praise be to Allah, Lord of the Universe."
(4) "The Merciful, the Compassionate."
(5) "Master of the Day of Judgment."
(6) "It is You we worship and You we ask for help."
(7) "Guide us to the straight path, the path of those upon whom you have bestowed favors, not of those against whom You are angry, nor of those who are lost."
It was reported by Muslim (also by Malik, at-Tirmidhi, Abu-Dawud, an-Nasa'i and Ibn Maja).