It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) in marfu' form that the Messenger of Allah (peace and blessings be upon him) said: "Recite the Qur'an with correct Arabic inflection (i'rab), for it is in the Arabic language."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8652