When the Messenger of Allah ﷺ recited the verse "Nor of those who go astray" (Surah al-Fatihah, verse 7), he would say Amin; and raised his voice (while uttering this word).  

كَانَ رَسُولُ اللَّهِ ﷺ إِذَا قَرَأَ { وَلاَ الضَّالِّينَ } قَالَ آمِينَ وَرَفَعَ بِهَا صَوْتَهُ  

Wail b, hujr said that he prayed behind the Messenger of Allah ﷺ,and he said Amin loudly and saluted at his right and left sides until I saw the whiteness of his cheek.  

أَنَّهُ صَلَّى خَلْفَ رَسُولِ اللَّهِ ﷺ فَجَهَرَ بِآمِينَ وَسَلَّمَ عَنْ يَمِينِهِ وَعَنْ شِمَالِهِ حَتَّى رَأَيْتُ بَيَاضَ خَدِّهِ  

When the Messenger of Allah ﷺ recited the verse "Not of those with whom Thou art angry, nor of those who go astray," he would say Amin so loudly that those near him in the first row would hear it.  

كَانَ رَسُولُ اللَّهِ ﷺ إِذَا تَلاَ { غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ } قَالَ آمِينَ حَتَّى يَسْمَعَ مَنْ يَلِيهِ مِنَ الصَّفِّ الأَوَّلِ  

Abu Hurairah reported the prophet ﷺ as saying; when the imam recites “ not of those with whom thou art angry, nor of those who go astray” (surah al-fatihah,verse 7) say Amin, for if one’s words (utterance of amin) synchronise with those of the angles, he will be forgiven his past sins.  

أَنَّ النَّبِيَّ ﷺ قَالَ إِذَا قَالَ الإِمَامُ { غَيْرِ الْمَغْضُوبِ عَلَيْهِمْ وَلاَ الضَّالِّينَ } فَقُولُوا آمِينَ فَإِنَّهُ مَنْ وَافَقَ قَوْلُهُ قَوْلَ الْمَلاَئِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ  

Abu Hurairah reported the Messenger of Allah (May peace be upon him) as saying; When the Imam says Amin, say Amin, for if anyone’s utterance of Amin synchronnises with that of the angles, he will be forgiven his past sins. Ibn shihab (al Zuhrl) said; The Messenger of Allah (May peace be upon him) used to say Amin (At the end of the Fatihah)  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا أَمَّنَ الإِمَامُ فَأَمِّنُوا فَإِنَّهُ مَنْ وَافَقَ تَأْمِينُهُ تَأْمِينَ الْمَلاَئِكَةِ غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ قَالَ ابْنُ شِهَابٍ وَكَانَ رَسُولُ اللَّهِ ﷺ يَقُولُ آمِينَ  

Messenger of Allah, do not say Amin before me.  

أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ لاَ تَسْبِقْنِي بِآمِينَ  

AbuMisbah al-Muqra'i said: We used to sit in the company of AbuZuhayr an-Numayri. He was a companion of the Prophet ﷺ, and he used to narrate good traditions. Once a man from among us made a supplication. He said: End it with the utterance of Amin, for Amin is like a seal on the book. AbuZuhayr said: I shall tell you about that. We went out with the Messenger of Allah ﷺ one night and came upon a man who made supplication with persistence. The Prophet ﷺ waited to hear him. The Prophet ﷺ said: He will have done something which guarantees (Paradise for him) if he puts a seal to it. One of the people asked: What should he use as a seal? He replied: Amin, for if he ends it with Amin, he will do something which guarantees (Paradise for him). Then the man who questioned the Prophet ﷺ came to the man who was supplicating, and said to him: So-and-so, end it with Amin and receive the good news. These are the words of Mahmud. Abu Dawud said: Al-Muqra'i is a clan of Himyar.  

كُنَّا نَجْلِسُ إِلَى أَبِي زُهَيْرٍ النُّمَيْرِيِّ وَكَانَ مِنَ الصَّحَابَةِ فَيَتَحَدَّثُ أَحْسَنَ الْحَدِيثِ فَإِذَا دَعَا الرَّجُلُ مِنَّا بِدُعَاءٍ قَالَ اخْتِمْهُ بِآمِينَ فَإِنَّ آمِينَ مِثْلُ الطَّابَعِ عَلَى الصَّحِيفَةِ قَالَ أَبُو زُهَيْرٍ أُخْبِرُكُمْ عَنْ ذَلِكَ خَرَجْنَا مَعَ رَسُولِ اللَّهِ ﷺ ذَاتَ لَيْلَةٍ فَأَتَيْنَا عَلَى رَجُلٍ قَدْ أَلَحَّ فِي الْمَسْأَلَةِ فَوَقَفَ النَّبِيُّ ﷺ يَسْتَمِعُ مِنْهُ فَقَالَ النَّبِيُّ ﷺ أَوْجَبَ إِنْ خَتَمَ فَقَالَ رَجُلٌ مِنَ الْقَوْمِ بِأَىِّ شَىْءٍ يَخْتِمُ قَالَ بِآمِينَ فَإِنَّهُ إِنْ خَتَمَ بِآمِينَ فَقَدْ أَوْجَبَ فَانْصَرَفَ الرَّجُلُ الَّذِي سَأَلَ النَّبِيَّ ﷺ فَأَتَى الرَّجُلَ فَقَالَ اخْتِمْ يَا فُلاَنُ بِآمِينَ وَأَبْشِرْ وَهَذَا لَفْظُ مَحْمُودٍ