What the person praying behind the imam should say
باب مَا يَقُولُ الْمَأْمُومُ
Sunan an-Nasa'i, Book of The At-Tatbiq (Clasping One's Hands Together)(view book)
It was narrated from Anas that:
The Prophet ﷺ fell from a horse onto his right side, and they entered upon him to visit him. The time for prayer came, and when he had finished praying he said: "The Imam is appointed to be followed, so when he bows, then bow, and when he stands up, then stand up, and when he says: 'Sami' Allahu liman hamidah (Allah hears the one who praises Him)' then say: 'Rabbana wa lakal-hamd (Our Lord, and to You be the praise).'"
أَخْبَرَنَا هَنَّادُ بْنُ السَّرِيِّ، عَنِ ابْنِ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ ﷺ سَقَطَ مِنْ فَرَسٍ عَلَى شِقِّهِ الأَيْمَنِ فَدَخَلُوا عَلَيْهِ يَعُودُونَهُ فَحَضَرَتِ الصَّلاَةُ فَلَمَّا قَضَى الصَّلاَةَ قَالَ " إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ " .
Classification
It was narrated that Rifa'ah bin Rafi said:
"We were praying behind the Messenger of Allah ﷺ one day and when he raised his head from bowing he said: 'Sami Allahu liman hamidah (Allah hears the one who praises Him).' A man behind him said: 'Rabbana wa lakal-hamd, hamdan kathiran tayyiban mubarakan fih. (O our Lord, and to You be praise, much blessed and pure praise.)' When the Messenger of Allah ﷺ had finished, he said: "Who is the one who spoke just now?" The man said: 'I did, O Messenger of Allah.' The Messenger of Allah ﷺ said: 'I saw thirty-some angels rushing to see which of them would write it down first.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ سَلَمَةَ، قَالَ أَنْبَأَنَا ابْنُ الْقَاسِمِ، عَنْ مَالِكٍ، قَالَ حَدَّثَنِي نُعَيْمُ بْنُ عَبْدِ اللَّهِ، عَنْ عَلِيِّ بْنِ يَحْيَى الزُّرَقِيِّ، عَنْ أَبِيهِ، عَنْ رِفَاعَةَ بْنِ رَافِعٍ، قَالَ كُنَّا يَوْمًا نُصَلِّي وَرَاءَ رَسُولِ اللَّهِ ﷺ فَلَمَّا رَفَعَ رَأْسَهُ مِنَ الرَّكْعَةِ قَالَ " سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ " . قَالَ رَجُلٌ وَرَاءَهُ رَبَّنَا وَلَكَ الْحَمْدُ حَمْدًا كَثِيرًا طَيِّبًا مُبَارَكًا فِيهِ . فَلَمَّا انْصَرَفَ رَسُولُ اللَّهِ ﷺ قَالَ " مَنِ الْمُتَكَلِّمُ آنِفًا " . فَقَالَ الرَّجُلُ أَنَا يَا رَسُولَ اللَّهِ . قَالَ رَسُولُ اللَّهِ ﷺ " لَقَدْ رَأَيْتُ بِضْعَةً وَثَلاَثِينَ مَلَكًا يَبْتَدِرُونَهَا أَيُّهُمْ يَكْتُبُهَا أَوَّلاً " .