(Second chain) The Messenger of Allah, peace and blessings be upon him, said: When the imam says "Sami' Allahu liman hamidah," you should say "Allahumma Rabbana lakal hamd." So whoever's words coincide with the words of the angels, his past sins will be forgiven.
Hadith Referenceالفتح الربانی / أبواب صفة الصلاة / 1712
Hadith Gradingمحدثین:صحیح
Hadith Takhrij«أخرجه البخاري: 796، 3228، ومسلم: 409، وانظر الحديث بالطريق الاول ، (انظر مسند أحمد ترقيم الرسالة: 9923 ترقیم بيت الأفكار الدولية: 9925»
Brief Explanation
Benefits: … One should not understand from this hadith that the follower (muqtadi) should not say "Sami‘a Allahu liman hamidah," because here the follower is not prohibited from saying this phrase; rather, the timing for saying "Rabbana wa laka al-hamd" for him has been specified. On the basis of the following evidences, everyone should say "Sami‘a Allahu liman hamidah": "Sami‘a Allahu liman hamidah" is included in the prophetic order of prayer, and there is no clear evidence indicating that it should not be recited while following the imam. The Noble Prophet sallallahu alayhi wa sallam himself said "Sami‘a Allahu liman hamidah" and said: ((Sallu kama ra’aytumoonee usalli.)) (Bukhari) … Pray as you have seen me pray. Sayyiduna Rifa‘ah ibn Rafi‘ radi Allahu anhu narrates that the Noble Prophet sallallahu alayhi wa sallam said: ((La tatimmu salatun li ahadin min al-nas hatta … yukabbira … thumma yarka‘a … thumma yaqulu Sami‘a Allahu liman hamidah hatta yastawee qa’iman.)) … The prayer of any person is not complete until he says the takbir … performs bowing (ruku‘) … and then says "Sami‘a Allahu liman hamidah" … (Abu Dawud, Hakim) This hadith is a clear text that without "Sami‘a Allahu liman hamidah" the prayer is not complete, therefore a clear evidence is required to prevent the followers from saying this phrase. The same applies to "wa la al-dallin" and "Amin," that the obligation of reciting Surah al-Fatihah behind the imam has already been established by other texts.