Sayyiduna Abu Sa‘id bin Ma‘la (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) passed by me and said: “Have you not read the statement of Allah Almighty: «يَاأَيُّهَاالَّذِينَآمَنُوااسْتَجِيبُوالِلَّهِ ...»[انفال: 24/8] meaning, when Allah and His Messenger call you, then respond with ‘yes’.” Then the Prophet (peace and blessings of Allah be upon him) said: “Shall I not teach you a surah before you leave the mosque, which is the greatest surah of the Qur’an?” Then when the Prophet (peace and blessings of Allah be upon him) intended to leave, he said: “«اَلْحَمْدُلِلَّهِرَبِّالْعَالَمِينَ» This is the seven oft-repeated verses and the Great Qur’an which has been given to you.”
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1531
By "Sab‘ al-Mathani" is meant the seven verses which are recited repeatedly, and this is an allusion to the noble verse: «﴿وَلَقَدْآتَيْنَاكَسَبْعًامِنَالْمَثَانِيوَالْقُرْآنَالْعَظِيمَ﴾ [الحجر : 87]». The details of this hadith are found in the narration of Sahih al-Bukhari.
Sayyiduna Abu Sa‘id radi Allahu anhu said that I was performing prayer when the Messenger of Allah sallallahu alayhi wa sallam called me in that very state, but I did not respond. Afterwards, when I presented myself in his service, I submitted that I was engaged in prayer. Upon this, he sallallahu alayhi wa sallam said: Did you not hear the command of Allah the Exalted... etc.