Umar bin al-Khattab (may Allah be pleased with him) says: Once, during the blessed lifetime of the Messenger of Allah (peace and blessings be upon him), I passed by Hisham bin Hakim bin Hizam (may Allah be pleased with him) while he was reciting Surah al-Furqan. I listened closely to his recitation, and he was reciting in many ways (huruf) which the Messenger of Allah (peace and blessings be upon him) had not taught me. I was about to stop him at once, but I waited for him to say the salam. When he said the salam, I put his own cloak around his neck and said: Who taught you this surah which I have just heard you reciting? He said: The Messenger of Allah (peace and blessings be upon him) taught it to me. I said: You are lying, by Allah! The Messenger of Allah (peace and blessings be upon him) taught me this very surah that you were reciting. With the cloak around his neck, I brought him to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, he is not reciting Surah al-Furqan the way you taught it to me. The Prophet (peace and blessings be upon him) said: "O Umar, let him go." Then he said: "Recite, Hisham." He recited it just as I had heard him recite. The Messenger of Allah (peace and blessings be upon him) said: "It was revealed like this." Then he said to me: "Recite." I recited it as the Messenger of Allah (peace and blessings be upon him) had taught me. The Prophet (peace and blessings be upon him) said: "It was revealed like this." Then the Messenger of Allah (peace and blessings be upon him) said: "This Qur'an has been revealed in seven modes (ahruf), so recite of it whichever is easy."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3986