عن أبي سعيد رافع بن المعلى رضي الله عنه قال: قال لي رسول الله صلى الله عليه وسلم : «أَلاَ أُعَلِّمُكَ أَعْظَمَ سُورَةٍ في القُرْآن قَبْلَ أنْ تَخْرُجَ مِنَ الْمَسْجِد؟» فَأخَذَ بِيَدِي، فَلَمَّا أرَدْنَا أنْ نَخْرُجَ، قُلْتُ: يَا رَسُولَ اللهِ، إنَّكَ قُلْتَ: لأُعَلِّمَنَّكَ أعْظَمَ سُورَةٍ في القُرْآنِ؟ قالَ: «الحَمْدُ للهِ رَبِّ العَالَمِينَ، هِيَ السَّبْعُ المَثَانِي وَالقُرْآنُ العَظِيمُ الَّذِي أُوتِيتُهُ».
[صحيح] - [رواه البخاري]
Abu Sa‘īd Rāfi’ ibn al-Mu‘alla (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) said to me: "Shall I teach you the greatest Sūrah in the Qur’an before you leave the mosque?" Then he took me by the hand, and when we were about to leave, I said: "O Messenger of Allah, you said: 'I shall teach you the greatest Sūrah in the Qur’an.'" He said: "It is 'Alhamdulillāh Rabb al-‘Ālāmīn.' (i.e. Sūrat al-Fātihah) It is the seven oft-repeated verses and the Glorious Qur’an which was given to me."
Sahih/Authentic. - [Al-Bukhari]
Abu Sa‘īd Rāfi‘ ibn Al-Mu‘alla,(may Allah be pleased with him) reported: "The Messenger of Allah (may Allah’s peace and blessings be upon him) said to me: 'Shall I': He used this form of speech to alert the addressed person to what comes after it. "Shall I teach you the greatest Sūrah in the Qur’an before you leave the mosque": He said so instead of teaching him the Sūrah directly in order to prompt him to clear his mind to receive this information with wholehearted readiness. "Then he took me by the hand": i.e. after he had said that, and we walked on. I said: "O Messenger of Allah, you said: 'I shall teach you the greatest Sūrah in the Qur’an.'" He said: "It is Alhamdulillāh Rabb al-‘Ālāmīn" (i.e. Sūrat al-Fātihah): Al-Fātihah is the greatest Sūrah because it encompasses all the purposes of the Qur’an and hence is called "Umm al-Qur’an" or 'the Mother of the Qur’an'. Then he mentioned what distinguishes Al-Fātihah from the other Sūrahs, thus making it the greatest of them all, saying: "It is As-Sab‘ al-Mathāni”. Mathāni is plural of "muthannah", meaning 'doubled', because it is recited twice in each Rak‘ah; or because it is followed by another Sūrah, so it is recited in a pair of Sūrahs; or because it includes two parts: praise and invocation; or because it includes eloquent meanings and skillful structure; or because it is repeated over the passage of time, so it will never cease to exist, and it is always taught, so it will never become extinct; or because its benefits are renewed with each state infinitely; or because it is derived from "thanā" or 'praise' because it includes praise of Allah, the Almighty, as if praising Him with His beautiful Names and Attributes; or because it is derived from "thanāya" because Allah has made it peculiar to this nation; and there are other possible meanings. "And the Glorious Qur’an": means that Al-Fātihah is also called 'the Glorious Qur’an' because it encompasses everything that is related to existence in this worldly life and in the afterlife as well as rulings and creeds.