Musnad Ahmad 830:53

'Abd al-Rahman ibn Mahdi narrated to us, from Hammad ibn Salamah, from 'Ali ibn Zayd, from 'Abd al-Rahman ibn Abi Bakrah, from his father, from the Prophet (peace and blessings of Allah be upon him), who said: "Jibril and Mika'il came to me. Jibril said: 'Recite the Qur'an upon one dialect.' Mika'il said: 'Ask him for more.' He said: 'Recite it upon seven dialects, all of which are sufficient and adequate, as long as you do not seal a verse of mercy with punishment, or a verse of punishment with mercy.'"