Musnad Ahmad 926:9

Abdullah narrated to us, Wahb ibn Baqiyyah narrated to me, Khalid ibn Abdullah narrated to us, from Isma'il—meaning Ibn Abi Khalid—from Abdullah ibn Isa, from Abd al-Rahman ibn Abi Layla, Ubayy ibn Ka'b narrated to me, he said: I was in the mosque when a man entered and prayed. He recited a recitation that I disapproved of. Then another man entered and prayed, and he recited a recitation different from the recitation of his companion. When we finished the prayer, we entered upon the Messenger of Allah (peace and blessings of Allah be upon him), and I said: "O Messenger of Allah, this one recited a recitation that I disapproved of, and this one entered and recited a recitation different from the recitation of his companion." The Messenger of Allah (peace and blessings of Allah be upon him) said to them: "Recite." So they recited, and he said: "You have done well." There fell into my soul a sense of denial, such as I had not felt even when I was in the Age of Ignorance. When the Messenger of Allah (peace and blessings of Allah be upon him) saw what had overcome me, he struck my chest. He said: I broke out in sweat, and it was as if I were looking at my Lord in fear. He said to me: "O Ubayy, my Lord sent to me and said: 'Recite upon one letter.' I replied to Him: 'Make it easy for my nation.' He replied to me: 'Recite upon two letters.' I replied to Him three times: 'Make it easy for my nation.' He replied to me: 'Recite upon seven letters, and for every time I replied to you, you have a request that I will grant you.' So I said: 'O Allah, forgive my nation. O Allah, forgive my nation.' And I delayed the third one for a day when all of creation will beseech me, even Ibrahim."