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«أم» means mother. «أم الكتاب» This Surah is named so because in the Noble Qur'an, writing begins with it. (For this reason, it is also called «فاتحة الكتاب» .) And in prayer, recitation also begins with it, and «الدين» means recompense, whether in good or in evil, as it is said: «كما تدين تدان‏.‏» (As you do, so shall you reap). Mujahid said that «الدين» means reckoning, while «مدينين‏» means «محاسبين‏» , that is, those who have been reckoned.

Narrated Abu Sa`id bin Al-Mu'alla: While I was praying in the Mosque, Allah's Messenger called me but I did not respond to him. Later I said, "O Allah's Messenger! 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 Qur'an?' He said, "Al-Hamdu-Li l-lah 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."