It is narrated from Hazrat Amr bin Aas (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: The Noble Qur'an has been revealed upon seven letters, so whichever letter you recite according to, you will recite correctly. Therefore, do not dispute about the Noble Qur'an, for disputing about the Qur'an is disbelief.