Musnad Ahmad 30:1271

Muhammad ibn Bishr narrated to us, Muhammad ibn ‘Amr narrated to us, Abu Salamah narrated to us, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Quran was revealed in seven ahruf (modes/dialects): 'Aliman (All-Knowing), Hakiman (All-Wise), Ghafuran (All-Forgiving), Rahiman (All-Merciful)."