What is the meaning of the hadith: (Do not do anything except with the Mother of the Qur'an)? And what is the ruling on reciting al-Fatihah behind the imam in the loud prayers?

The questioner : O Shaykh, is it authentic from the Prophet (peace and blessings be upon him) that he said to the Companions—meaning after the prayer had ended: . They said: Yes. He said: ; is this hadith authentic?

The Shaykh : There is a point of consideration regarding the authenticity of this hadith, and that is from two aspects:
The first aspect is regarding the chain of narration; for it hinges on Makhul al-Shami, and Makhul al-Shami is trustworthy; however, he has been accused of tadlis (concealment of a defect in a chain). What occurred in his narration of this hadith is that sometimes he narrates it from his shaykh using the "an" (from) formula, and sometimes he narrates it through an intermediary, explicitly stating in that case that he heard it from the intermediary. As for when he omits the intermediary, he uses the "an" formula, and when he mentions the intermediary, he explicitly states the narration. The intermediary is Mahmud, and this Mahmud is unknown; so when he explicitly states the narration, Makhul says: "Mahmud narrated to me," so the defect is the unknown narrator. And when he omits Mahmud from the chain, he uses the "an" formula and does not explicitly state the narration; this is the defect of this hadith regarding the chain of narration.
Then, the hadith has been narrated, indeed established with a sound chain, through another route in "Musnad al-Imam Ahmad" (may Allah have mercy on him) with the wording: ; in that case, the hadith enters a different subject matter than the subject of the obligation of the follower reciting al-Fatihah behind the imam in the loud prayer; for according to the narration of Makhul: , this narration, if it were authentic, would only indicate that recitation behind the imam is not obligatory, but rather permissible. This is because among the principles of Islamic jurisprudence is that "permission for something after prohibition of it does not imply its obligation, but only its permissibility." So here, as you see, he prohibited them with an absolute prohibition; ; do not recite anything behind the imam except al-Fatihah. So "except" is an exception from the prohibition, and an exception from a prohibition does not mean the obligation of the excepted item, but rather means the permissibility of doing it or reciting it, as is the case here.
The evidence for this mentioned rule is found in the books of the principles of jurisprudence, and the most famous of it is His saying (Glorified and Exalted is He): ; so His saying (Mighty and Majestic): does not mean the obligation of hunting after exiting the state of ihram—and upon you be peace and the mercy of Allah and His blessings—likewise in what we are discussing, "do not do" is a prohibition, "except with the opening of the Book" is an exception; so it indicates that the exception is only permissible and not that it is obligatory. This is confirmed by the other narration in "Musnad Ahmad" with the sound chain: ; this expression is very precise and confirms that the permission to recite al-Fatihah in the loud prayer is not for the sake of obligation, but for the sake of permissibility; indeed, it is the weaker position of permissibility. This is on the same scale as His saying (Glorified and Exalted is He) in the Qur'an: , so no one says that the taqiyyah (dissimulation) mentioned in this verse is obligatory, but rather it indicates permissibility, and the weaker position of permissibility. Otherwise, it is better for a person to stand firm and speak out for the truth, as Bilal al-Habashi (may Allah be pleased with him) did when they tortured him and offered him that he disbelieve in his Messenger; so he refused and persisted, while 'Ammar ibn Yasir (may Allah have mercy on him) took the dispensation and said the word—if the narration about him is authentic—until they released him. So His saying (Mighty and Majestic): does not mean the obligation of taqiyyah, but rather indicates that it is permissible, and permissibility of the weaker position. Likewise, recitation behind the imam in the loud prayer in his saying (peace and blessings be upon him): , so that means it is permissible for him; but it is better that he does not recite. This is what the Shari'ah settled upon later, as came in the hadith of Abu Hurayrah: "So the Companions of the Prophet (peace and blessings be upon him) stopped reciting behind the Prophet (peace and blessings be upon him) in what the Messenger of Allah (peace and blessings be upon him) would recite loudly." And in that case, the ruling of the Shari'ah has settled in accordance with His saying (Glorified and Exalted is He): .
This is the answer.