Abu Sa`id Ar-Rafi` bin Al-Mu`alla (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "Shall I teach you the greatest Surah in the Quran before you leave the mosque?" Then he (PBUH), took me by the hand, and when we were about to step out, I reminded him of his promise to teach me the greatest Surah in the Quran. He (PBUH) said, "It is `Alhamdu lillahi Rabbil `Alamin (i.e., Surat Al-Fatihah) which is As-Sab` Al-Mathani (i.e., the seven oft-repeated Ayat) and the Great Quran which is given to me."[Al-Bukhari].
Allah the Almighty says:
"And indeed, We have bestowed upon you seven of Al-Mathani (the seven repeatedly recited Ayat), (i.e., Surat Al-Fatihah) and the Grand Qur'an." (15:87).
This Hadith is an elaboration of the Quranic Ayah quoted above. The Prophet (PBUH) held that As-Sab` Al-Mathani is Surat Al-Fatihah (the Opening Chapter) because these seven Ayat are recited not only in every Salat but in every Rak`ah of every Salat for the reason that no Salat is valid without the recitation of this Surah, as is evident from the saying of the Prophet (PBUH): "The Salat of that person who has not recited Surat Al-Fatihah is void." It has also been regarded as the greatest Surah of the Qur'an as it covers all the objects of the Qur'an and is a condensation of all the subjects which have been discussed in detail in its other Surah. It includes the belief in the Tauhid (the belief in the Oneness of Allah), the worship of Allah, the Day of Resurrection, promise (of Jannah) and warning (against Hell-fire), instances of the pious and impious people of the past communities, and the lessons their conduct has for others. Abu Dawud and At-Tirmidhi have reported a Hadith in which the Messenger of Allah (PBUH) said: "Al-hamdu lillahi Rabbil `Alamin (i.e., Surat Al-Fatihah) is Umm-ul-Qur'an that is its origin, basis and essence".