In a Hadith Qudsi, the Prophet has narrated what God says when a person recites the surat al-Fatiha.
The person says, "In the Name of God, the All-Merciful, the Compassionate", and God says, "My servant mentions Me". The servant then says, "Praise be to Allah, the Lord of the Worlds," and God says in his turn, "My servant lends Me grace." The servant then says "The All-Merciful, The Compassionate, " and God replies "My servant praises Me." The servant says, "Master of the Day of Judgement," and God says, "My servant glorifies Me and submits himself to Me."
This first half of this surah relates exclusively to God and the servant's invocation of Allah's attributes. In the next half of the surah, the servant prays with a complete sense of humility.
The servant says, "You alone do we worship and unto You alone do we turn for help," and God says, "This is shared between Me and My servant, and My servant will receive that which he asks." When the servant says, "Guide us to the Straight Path, The path of those whom You have favoured, Not those who have incurred Your wrath, nor those who are astray," God says, "All that comes back to My servant, and My servant will receive that for which he asks." Thus, the second half of this surah is related exclusively to the believer.
It is because of this mutual participation between the Divine and the human in this surah, which is considered to be the heart of the Holy Qur'an, that the canonical prayer is regarded as not having been performed if this Surah is not recited.