Sahih Al Bukhari – Book of Prophetic Commentary On The Qur'an (Tafseer Of The Prophet (pbuh))
Volumn 006, Book 060, Hadith Number 001.
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Narated By Abu Said bin Al-Mu'alla RA: While I was praying in the Mosque, Allah's Apostle PBUH called me but I did not respond to him. Later I said, "O Allah's Apostle! I was praying." He said, "Didn't Allah say'… "Give your response to Allah (by obeying Him) and to His Apostle when he calls you." (8.24)

He then said to me, "I will teach you a Sura which is the greatest Sura in the Qur'an, before you leave the Mosque." Then he got hold of my hand, and when he intended to leave (the Mosque), I said to him, "Didn't you say to me, 'I will teach you a Sura which is the greatest Sura in the Quran?' He said, "Al-Hamdu-Lillah Rabbi-l-Alamin (i.e. Praise be to Allah, the Lord of the worlds) which is Al-Sab'a Al-Mathani (i.e. seven repeatedly recited Verses) and the Grand Qur'an which has been given to me."

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