The Messenger of Allah (peace and blessings be upon him) commands you that each of you recite as he was taught

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: The Messenger of Allah (peace and blessings be upon him) taught me Surah (Ha Mim). Then in the evening I went to the mosque, and a group was sitting there. I said to one man among them: Recite (this surah) to me. He was reciting it with letters (words) that I did not recite. I asked him: Who taught it to you? He said: The Messenger of Allah (peace and blessings be upon him) taught it to me. Then we went to the Messenger of Allah (peace and blessings be upon him), and a man was sitting with him. I said: We have differed in our recitation. On hearing this, the colour of the blessed face of the Messenger of Allah (peace and blessings be upon him) changed, and when I mentioned the difference he disliked it (in his heart). Then he (peace and blessings be upon him) said: "It was only differing (in religion) that destroyed those before you." Then he (peace and blessings be upon him) whispered to Sayyiduna Ali (may Allah be pleased with him), and Sayyiduna Ali (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) commands you that each one of you should recite just as he was taught. So we went away, each one of us reciting letters that his companion did not recite.

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) with the same chain, and in it there is the addition: We went to the Messenger of Allah (peace and blessings be upon him), and a man was sitting with him. Zirr (the narrator) says: People used to revile him! That is, Sayyiduna Ali (may Allah be pleased with him).
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it with this wording.

It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him) that he said: Recitation (reading the Qur'an) is a sunnah that must be followed. Sulayman (the narrator) says: It means that you should not oppose the people with your own opinion in the matter of following.
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: For several years in Makkah we recited the Mufassal surahs, in which there was no call of "O you who believe!"
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to me: "Indeed, Allah Almighty has commanded me to recite the Qur'an to you." Then he (peace and blessings be upon him) recited: "Those who disbelieved among the People of the Book and the polytheists were not" [Al-Bayyinah: 1], and among its descriptions (or verses) was also this: "If the son of Adam were to have a valley of wealth given to him, he would ask for a second; and if he were given a second as well, he would ask for a third. And nothing fills the belly of the son of Adam except dust, and Allah accepts the repentance of those who repent. And the true religion in the sight of Allah is 'Hanifiyyah' (the religion of pure devotion), which is neither Judaism nor Christianity; and whoever does any good deed, it will never be left unappreciated."
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he said: Once, while I was walking along one of the roads of Madinah reciting a verse of the Book of Allah, the Mighty and Majestic, suddenly a man called out from behind: "Follow along, Ibn Abbas," and I saw that it was the Commander of the Believers, Sayyiduna Umar (may Allah be pleased with him). I said: I will take you to Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) (so that he may confirm this recitation). He asked: Did he himself teach it to you the way I heard you reciting? I said: Yes. He sent a messenger with me and said: Go with him to Ubayy bin Ka'b and see whether Ubayy also recites it this way. The messenger and I went to Sayyiduna Ubayy bin Ka'b, and I said: O Ubayy! I was reciting a verse of the Book of Allah when Umar bin al-Khattab (may Allah be pleased with him) called out from behind, "Follow along, Ibn Abbas," and I said that I would take him to Ubayy bin Ka'b, so he has sent his messenger with me. So did you teach me this verse just as I recited it? Sayyiduna Ubayy said: Yes. The messenger went back, and I went off to attend to my need. Then Sayyiduna Umar (may Allah be pleased with him) himself came to Sayyiduna Ubayy. He had just finished washing his head, and his slave-girl was grooming his beard with a wooden comb. Ubayy said: Welcome, O Commander of the Believers! Have you come to visit, or for some need? Umar (may Allah be pleased with him) said: Rather, I have come for a need. He sat down along with his two freed slaves, until he (Ubayy) had finished grooming his beard and the right side of his hair had been combed and then the left side. After finishing, he turned towards Umar and asked: What is the need of the Commander of the Believers? Umar (may Allah be pleased with him) said: O Ubayy! Why do you make people despair (by reciting variant letters to them)? Ubayy replied: O Commander of the Believers! I received the Qur'an from the one who received it from Jibril (peace be upon him) while it was completely fresh. At this Umar (may Allah be pleased with him) said three times: "By Allah! Neither are you going to desist, nor am I going to bear it patiently." Then he got up and left.
This hadith has an authentic chain (sahih al-isnad), but the two Shaykhs (Bukhari and Muslim) did not record it.

It is narrated from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that he used to recite (a verse of Surah al-Fath) this way: "When those who disbelieved put in their hearts zealotry, the zealotry of the time of ignorance; and had you shown zealotry as they did, the Sacred Mosque would have been ruined; then Allah sent down His tranquillity upon His Messenger" [Surah al-Fath: 26]. This reached Sayyiduna Umar (may Allah be pleased with him) and he found it very displeasing. He sent for Sayyiduna Ubayy while he was applying medicine to one of his camels. He came to him, and Umar (may Allah be pleased with him) called a group of his companions, among whom was Sayyiduna Zayd bin Thabit (may Allah be pleased with him). Then Umar (may Allah be pleased with him) asked: Which of you recites Surah al-Fath? Sayyiduna Zayd recited it just as we recite it today. Umar (may Allah be pleased with him) showed sternness over this (variant recitation). Then Ubayy said: May I speak? He said: Speak. Ubayy said: You know that I used to go in to the Prophet (peace and blessings be upon him) and he would teach me the Qur'an while you people were at the door. Now if you wish that I teach the people just as I was taught, well and good; otherwise I will not teach a single letter for as long as I live. Sayyiduna Umar (may Allah be pleased with him) said: "No, rather, teach the people (in the same way)."
This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it.

It is narrated from Sayyiduna Jundub (may Allah be pleased with him) that he said: I came to Madinah to acquire knowledge. When I entered the mosque of the Messenger of Allah (peace and blessings be upon him), I saw people sitting in circles engaged in learned conversation. I went forward and reached a circle in which there was a man of pale complexion wearing two sheets, who looked like a traveller. I heard him saying three times: "By the Lord of the Ka'bah! The people of the pact (those in authority) have perished, and I feel no grief for them." I sat down with him, and when he finished his talk and stood up, I asked about him. The people said: This is the leader of the people, Sayyiduna Ubayy bin Ka'b. I followed him to his house, and I saw that his house, his clothing and his outward state were all extremely simple and worn. I greeted him with salam and he returned it. Then he asked me: Where are you from? I said: I am from the people of Iraq. At this he became angry and said: (The people of Iraq) are the ones who ask the most questions. I turned towards the qiblah, knelt on my knees, raised my hands and said: O Allah! We bring our complaint about them (the scholars) to You. We spend our wealth in the pursuit of knowledge, we wear out our bodies and bear the hardships of travel, but when we meet them they treat us with sullen faces. On hearing this, Sayyiduna Ubayy bin Ka'b began to weep and began trying to placate me, saying: Woe to you! I did not mean that. Then he said: I promise you that if I remain alive until Friday, I will certainly relate what I heard from the Messenger of Allah (peace and blessings be upon him), and in it I will not care for the blame of any blamer. Then I left and began waiting for Friday. When Thursday came, I saw that the streets of Madinah were filled with people. I asked: What has happened? The people said: We think you are a stranger. I said: Yes. They said: The leader of the people, Ubayy bin Ka'b, has passed away. Then when I met Sayyiduna Abu Musa al-Ash'ari in Iraq and related this incident to him, he said: If only he had lived until that word of his had reached us.
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.