Bukhari

Hadith Commentary:
The seven verses of Surah Al-Fatihah are recited repeatedly in every obligatory prayer.
It is necessary for every imam and follower (muqtadi) to recite them, and without their recitation, the prayer is not valid.
For this reason, this surah is called "Sab‘ al-Mathani" (the Seven Oft-Repeated) and "Qur’an al-‘Azim" (the Great Qur’an).
The statement of those who say that it is impermissible to recite Surah Al-Fatihah behind the imam is incorrect.

Hadith Commentary:
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Surah Al-Fatihah consists of seven verses which are recited repeatedly in every prayer. It is obligatory to recite Surah Al-Fatihah in every unit (rak‘ah) of the prayer, whether one is the imam or a follower, and whether the prayer is obligatory (fard) or supererogatory (nafl); without reciting Surah Al-Fatihah, the prayer is not valid, as is explicitly stated in other ahadith.

➋
Surah Al-Fatihah is called the “Great Qur’an” (Qur’an ‘Azim) because it contains the summary of the entire Qur’an’s teachings, as if the ocean has been confined within a vessel. First, there is the recognition of Allah and His praise and glorification; then, a comprehensive mention of reward and punishment on the Day of Judgment; after that, an affirmation of complete avoidance of all forms of shirk (associating partners with Allah) and a pledge to seek all kinds of help only from Allah; finally, the determination of the straight path (sirat al-mustaqim) and the request and supplication to adopt it. These are the themes that are mentioned in the Qur’an in both concise and detailed manners, and various styles have been adopted for their exposition. In fact, the further summary of this surah is in the verse: ﴿إِيَّاكَ نَعْبُدُ وَإِيَّاكَ نَسْتَعِينُ﴾ (“You alone we worship and You alone we ask for help”). There is no room here for further elaboration.
And Allah is the One whose help is sought.

Explanation:
1:
The author has mentioned this hadith in the commentary of the statement of Allah, the Exalted:
﴿وَلَقَدْ آتَيْنَاكَ سَبْعًا مِّنَ الْمَثَانِي وَالْقُرْآنَ الْعَظِيمَ﴾ (Al-Hijr: 87).

1457. Commentary: [اُم]
It means "origin" or "foundation." Since this blessed surah is a summary of the subjects of the Qur'an—especially monotheism (tawhid al-uluhiyyah, rububiyyah, and names and attributes), prophethood, and the Hereafter—it has been named Umm al-Qur'an and Umm al-Kitab. And "al-sab‘ al-mathani" refers to those seven verses which are repeatedly recited. In Surah al-Hijr, verse 87, it is stated: [وَلَقَدْ آتَيْنَاكَ سَبْعًا مِّنَ الْمَثَانِي وَالْقُرْآنَ الْعَظِيمَ]
"Indeed, We have given you seven verses which are repeatedly recited, and the magnificent Qur'an."