The Messenger of Allah ﷺ said, "The imam is appointed only to be followed. When he says the takbir, say the takbir. When he recites, listen in silence. When he says, 'not of those who have earned anger, nor of those who go astray' [Fatihah 1:7], say 'Ameen.' When he bows, bow with him. When he says, 'Allah hears whoever praises Him,' say, 'Our Lord, to You belongs all praise.' When he prostrates, prostrate. And when he prays sitting, all of you pray sitting."
قال رسول الله ﷺ: إنما جعل الإمام ليؤتم به، فإذا كبر فكبروا، وإذا قرأ فأنصتوا، وإذا قال: غير المغضوب عليهم ولا الضالين، فقولوا: آمين، وإذا ركع فاركعوا، وإذا قال: سمع الله لمن حمده، فقولوا: اللهم ربنا ولك الحمد، وإذا سجد فاسجدوا، وإذا صلى جالسا فصلوا جلوسا.
Narrated by Abu Dawud (604) in abbreviated form, Nasai (921), and Ibn Majah (846), whose wording is used here, all of them through the route of Abu Khalid Ahmar, from Ibn Ajlan, from Zayd bin Aslam, from Abu Salih, from Abu Hurairah, who then related the hadith. Abu Dawud said, "This addition, 'and when he recites, be silent,' is not preserved, and in my view the error lies with Abu Khalid." End of quote. For more, see Al-Qiraah Khalfa Al-Imam by Bayhaqi (pp. 133-134).I say, the scholars have differed regarding this addition. Major hadith authorities such as Bukhari, Abu Dawud, Abu Hatim, Ibn Main, Ibn Khuzayma, and others held that it is not sound. Ibn Abu Hatim reported from his father that he said, "This phrase is not preserved, it is one of the confusions of Ibn Ajlan."Among those who graded this addition sahih was Imam Muslim, may Allah have mercy on him. He was asked, "Is it sahih?" He said, "Yes." It was said to him, "Then why did you not include it here?" He replied, "Not everything I consider sahih have I placed here, I have only placed here what they agreed upon." Sahih Muslim (1/304).As for Abu Dawud's statement that the error, in his view, lies with Abu Khalid, Munziri and others responded to him by noting that Abu Khalid, who is Sulayman bin Hayan, is trustworthy, and both Bukhari and Muslim relied on him as evidence. Moreover, he was not alone in narrating it, for he was corroborated by Muhammad bin Saad Ansari, who is considered trustworthy by Nasai (922). Nasai said, "Makhrami used to say he is trustworthy," meaning Muhammad bin Saad Ansari.Assuming this addition is established, we must interpret the hadith so that its parts do not contradict one another. His saying, "and when he recites, be silent," means listen to his recitation and do not recite while he is reciting, but when he pauses, then recite, for it has been established that the Prophet ﷺ used to have two pauses of silence, one pause after the takbir and another pause after reciting Surah Fatihah. The author of Al-Mughni favored this view, and he reported from Abu Salama bin Abdul-Rahman, "The imam has two pauses of silence, so make use of them to recite the Opening of the Book." Al-Mughni (1/266).I say, this is one of the best ways of reconciliation, bringing together the sahih hadiths concerning the recitation of Surah Fatihah.