Sayyiduna Abu Sa‘id bin al-Mu‘alla al-Ansari (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) passed by me (and in al-Bukhari: I was praying, and the Messenger of Allah (peace and blessings be upon him) called me, but I did not respond). So the Messenger of Allah (peace and blessings be upon him) said: “Did Allah not say: «﴿يَاأَيُّهَاالَّذِينَآمَنُوااسْتَجِيبُوالِلَّهِوَلِلرَّسُولِإِذَادَعَاكُمْ . . . . . .﴾»[انفال: 24/8] ‘O you who believe! Respond to Allah and to the Messenger when he calls you’?” Then the Messenger of Allah (peace and blessings be upon him) said: “Before you leave the mosque, I will tell you about the greatest surah in the Qur’an.” And when the Messenger of Allah (peace and blessings be upon him) intended to leave, he said: “That greatest surah is ‘Alhamdu lillahi Rabbil ‘alamin’ (i.e., Surah al-Fatihah), which is the Seven Oft-Repeated Verses and the Great Qur’an which has been given to you.”
Hadith Referenceسنن دارمي / من كتاب فضائل القرآن / 3403
«سبع المثاني» These are the seven verses which are repeatedly recited in prayer, and without them, no rak‘ah is valid. It is obligatory for everyone in prayer—whether the imam, the follower (muqtadi), or the individual praying alone (munfarid)—to recite them. This is the Great Qur’an.