Rabi' (may Allah have mercy on him) said: I heard al-Buwayti (may Allah have mercy on him) saying: Whoever says that the Qur'an is created is a disbeliever. Allah Almighty says: "When We intend a thing, Our only word to it is that We say to it: 'Be,' and it is." [Surah al-Nahl: 40] Allah Almighty has informed us that He creates His creation by the word "Kun" (Be); so whoever claims that the word "Kun" is created has claimed that Allah creates creation by means of something created.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20897