Musnad Ahmad 507:3

Abu Salamah al-Khuza'i narrated to us, Sulayman ibn Bilal narrated to us, Yazid ibn Khusayfah narrated to me, Busr ibn Sa'id informed me, he said: Abu Juhaym narrated to me: That two men differed regarding a verse of the Qur'an. One said: "I received it from the Messenger of Allah, may Allah bless him and grant him peace." The other said: "I received it from the Messenger of Allah, may Allah bless him and grant him peace." So they asked the Prophet, may Allah bless him and grant him peace, and he said: "The Qur'an is recited upon seven ahruf (modes/dialects), so do not dispute regarding the Qur'an, for disputing in the Qur'an is disbelief."