Musnad Ahmad 926:1

Yahya ibn Sa'id narrated to us, from Isma'il ibn Abi Khalid, [who said]: 'Abdullah ibn 'Isa narrated to me, from 'Abd al-Rahman ibn Abi Layla, from Ubayy ibn Ka'b, who said: I was in the mosque when a man entered and recited a recitation which I disapproved of. Then another entered and recited a recitation different from the recitation of his companion. We all stood up and entered upon the Messenger of Allah (peace and blessings of Allah be upon him). I said: "O Messenger of Allah, this one recited a recitation which I disapproved of, then this one entered and recited a recitation other than the recitation of his companion." The Prophet (peace and blessings of Allah be upon him) said to them: "Recite." So they recited. He said: "You have both done correctly." When the Prophet (peace and blessings of Allah be upon him) said to them what he said, it weighed heavily upon me, [more so] than when I was in the Age of Ignorance. When he saw what had overcome me, he struck my chest, and I broke out in sweat, and it was as if I were looking at Allah in fear. He said: "O Ubayy, my Lord sent to me: 'Recite the Qur'an upon one dialect.' I replied to Him: 'Make it easy for my nation.' So He sent to me: 'Recite it upon two dialects.' I replied to Him: 'Make it easy for my nation.' So He sent to me: 'Recite it upon seven dialects, and for every reply you have a request you may ask of Me.'" He said: "I said: 'O Allah, forgive my nation. O Allah, forgive my nation.' And I delayed the third [request] for a day when all creation will beseech me, even Ibrahim."