He who observed prayer but he did not recite the Umm al-Qur'an in it, and the rest of the hadith is the same as transmitted by Sufyan, and in this hadith the words are:" Allah the Most High said: the prayer is divided into two halves between Me and My servant. The half of it is for Me and the half of it is for My servant."
The Book of Prayers
Sahih Muslim 395 b
Hadith Commentary
This noble hadith qudsi establishes the profound spiritual reality of the prayer (salah) as a direct communion between the servant and his Lord. The recitation of al-Fatihah (Umm al-Qur'an) is made obligatory, for it is through these divine words that the conversation is initiated.
The division of the prayer into two halves signifies a sacred covenant. The first half, belonging to Allah, comprises the servant's praise, glorification, and submission. The second half, for the servant, contains Allah's response, guidance, and acceptance. Thus, the prayer becomes a complete cycle of giving and receiving.
He who omits the Mother of the Book has invalidated his prayer, for he has failed to establish the very foundation upon which this divine dialogue is built. The completeness of one's devotion is measured by the completeness of one's recitation.