"The Imam is appointed to be followed. When he bows, then bow, when he stands up, then stand up, when he prostrates, then prostrate, and when he says Sami' Alldhu liman hamidah (Allah hears the one who praises Him), then say, Rabbanri lakal-hamd (Our Lord, to You be the praise)." (Using translation from Nasāʾī 794)  

أَنَّ النَّبِيَّ ﷺ سَقَطَ مِنْ فَرَسٍ عَلَى شِقِّهِ الْأَيْمَنِ فَدَخَلُوا عَلَيْهِ يَعُودُونَهُ فَحَضَرَتِ الصَّلَاةُ فَلَمَّا قَضَى الصَّلَاةَ قَالَ إِنَّمَا الْإِمَامُ لِيُؤْتَمَّ بِهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا رَفَعَ فَارْفَعُوا وَإِذَا قَالَ سَمِعَ اللهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا وَلَكَ الْحَمْدُ  

One day we were praying behind the Messenger of Allah ﷺ. When the Messenger of Allah ﷺ raised his head after bowing, he said: Allah listened to him who praised Him. A man behind the Messenger of Allah ﷺ said: O Allah, Our Lord, and to Thee be praise, much praise, good and blessed. When the Messenger of Allah ﷺ finished his prayer, he asked: Which of you if the one who spoke (the words) just now. The man said: I (uttered) these words, Prophet of Allah. The Messenger of Allah ﷺ said: I saw more than thirty angels racing against one another to be the one to write them first. (Using translation from Abū Dāʾūd 770)  

كُنَّا يَوْمًا نُصَلِّي وَرَاءَ رَسُولِ اللهِ ﷺ فَلَمَّا رَفَعَ رَأْسَهُ مِنَ الرَّكْعَةِ قَالَ «سَمِعَ اللهُ لِمَنْ حَمِدَهُ» فَقَالَ رَجُلٌ وَرَاءَهُ رَبِّنَا وَلَكَ الْحَمْدُ حَمْدًا كَثِيرًا طَيِّبًا مُبَارَكًا فِيهِ فَلَمَّا انْصَرَفَ رَسُولُ اللهِ ﷺ قَالَ «مِنَ الْمُتَكَلِّمُ آنِفًا؟» قَالَ رَجُلٌ أَنَا يَا رَسُولَ اللهِ قَالَ رَسُولُ اللهِ ﷺ «لَقَدْ رَأَيْتُ بِضْعَةً وَثَلَاثِينَ مَلِكًا يَبْتَدِرُونَهَا أَيُّهُمْ يَكْتُبُهَا أَوَّلُ؟»