Musnad Ahmad 830:141

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Ali ibn Zayd informed us, from 'Abd al-Rahman ibn Abi Bakrah, from Abu Bakrah: "Gabriel, peace be upon him, said: 'O Muhammad, recite the Qur'an in one mode.' Michael, peace be upon him, said: 'Ask him for more.' So he asked him for more, and he said: 'Recite it in two modes.' Michael said: 'Ask him for more.' So he asked him for more, until it reached seven modes. He said: 'Each of them is healing and sufficient, as long as you do not seal a verse of punishment with mercy, or a verse of mercy with punishment, similar to your saying: "Come here" (ta'ala) and "Approach" (aqbil), and "Hasten" (halumma) and "Go" (idhhab), and "Hurry" (asri') and "Make haste" ('ajil).'"