Sahih Muslim, Chapter: Prayers (Kitab Al-Salat)

that he had heard the Messenger of Allah (may peace be upon him) say: 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."