[How should the Qur’an be recited in prayer?]

Answer (and Allah grants success): The Qur’an came in the language of the Arabs, as He, Exalted is He, says: “A Qur’an in Arabic, without any crookedness.” [Az-Zumar:28]. On this basis, it is necessary that the Qur’an be recited in prayer in the manner characteristic of Arabic speech in their dialogues, addresses, orations, and poetry. Beyond that, forms of beautification are supererogatory—namely: articulating the letters from their points of articulation, and pronouncing the words properly inflected without departing from the rules of the language. As for lengthening the vowels (madd), idghām, and ikhfāʾ, they are not required.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1