Hadith Qudsi 8:

Narrated Abu Hurairah: the Prophet said:

A prayer performed by someone who has not recited the Essence of the Qur’an during it is deficient (and he repeated the word three times), incomplete.

Someone said to Abu Hurairah: "[Even though] we are behind the imam?" He said: Recite it to yourself, for I have heard the Prophet say:

Allah (mighty and sublime be He), had said: ‘I have divided prayer between Myself and My servant into two halves, and My servant shall have what he has asked for. When the servant says: ‘Al-hamdu lillahi rabbi l-alamin,’ Allah (mighty and sublime be He) says: ‘My servant has praised Me.’ And when he says: ‘Ar-rahmani r-rahim,' Allah (mighty and sublime be He) says: ‘My servant has extolled Me,’ and when he says: ‘Maliki yawmi d-din,’ Allah says: ‘My servant has glorified Me’ - and on one occasion He said: ‘My servant has submitted to My power.’ And when he says: ‘Iyyaka na budu wa iyyaka nasta in,’He says: ‘This is between Me and My servant, and My servant shall have what he has asked for.’ And when he says: ‘Ihdina s-sirata l- mustaqim, siratal ladhina an amta alayhim ghayril-maghdubi alayhim wa la d-dallin’, He says: ‘This is for My servant, and My servant shall have what he has asked for.’

It was related by Muslim (also by Malik, at-Tirmidhi, Abu-Dawud, an-Nasa'i and Ibn Majah).