Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) narrates: I was sitting in the mosque when a man entered and recited, and I found his recitation unfamiliar. Then another man came and recited in a way different even from the first one's recitation. When we left there, we went to the Messenger of Allah (peace and blessings be upon him). I said: O Messenger of Allah, this man recited in a way that I object to, and this other one recited differently even from his companion's recitation.
The Messenger of Allah (peace and blessings be upon him) said to that man: "Recite," and he recited. Then he said to the other: "Recite," and he too recited. Then the Messenger of Allah (peace and blessings be upon him) said: "Both of you have recited correctly." When I saw that the Messenger of Allah (peace and blessings be upon him) had approved of both of them, many misgivings arose in my heart, and I began to wish that I were at that moment in the Jahiliyyah (and had embraced Islam after it). When the Messenger of Allah (peace and blessings be upon him) saw my condition, he struck my chest with his blessed hand; I broke into a sweat, and it was as if I were looking at Allah Almighty. Then he (peace and blessings be upon him) said: "O Ubayy bin Ka'b, my Lord sent word to me (sent revelation) that I should recite the Qur'an in one mode, but I sent the angel back and said: O my Lord, make it easy for my Ummah. He repeated the same thing a second time: recite in one mode. I said: O my Lord, make it easy for my Ummah. Then the third time the reply came to me: recite the Qur'an in seven modes, and for every time you sent it back you have a supplication that you may make to Me. I said: O Allah, forgive my Ummah; O Allah, forgive my Ummah. And I have deferred the third supplication until the Day of Resurrection, the day on which all people will turn to me, even Sayyiduna Ibrahim (peace be upon him)."
In the narration of Sahih Muslim: There arose in my heart a thought of denial such as I had not had even when I was in the Jahiliyyah. Another narration has: "Doubts arose in my heart, and this weighed on me so heavily that nothing like it had weighed on me even in the days of the Jahiliyyah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3987