صحيح ابن خزيمه
Sahih Ibn Khuzaymah
جماع أبواب الأذان والإقامةChapters on Adhan and Iqamah
(140) بَابُ الْجَهْرِ بِآمِينَ عِنْدَ انْقِضَاءِ فَاتِحَةِ الْكِتَابِ فِي الصَّلَاةِ الَّتِي يَجْهَرُ الْإِمَامُ فِيهَا بِالْقِرَاءَةِ.Chapter 140: Saying "Ameen" aloud after Al-Fatihah in prayers where the imam recites aloud
Hadith 572
نا مُحَمَّدُ بْنُ يَحْيَى ، نا أَبُو سَعِيدٍ الْجُعْفِيُّ ، حَدَّثَنِي ابْنُ وَهْبٍ ، أَخْبَرَنِي أُسَامَةُ وَهُوَ ابْنُ زَيْدٍ ، عَنْ نَافِعٍ ، عَنِ ابْنِ عُمَرَ ، كَانَ " إِذَا كَانَ مَعَ الإِمَامِ يَقْرَأُ بِأُمِّ الْقُرْآنِ فَأَمَّنَ النَّاسُ ، أَمَنَّ ابْنُ عُمَرَ ، وَرَأَى تِلْكَ السُّنَّةَ "
It is narrated from Hazrat Nafi' (may Allah have mercy on him) that when Sayyiduna Ibn Umar (may Allah be pleased with them both) was with the Imam and he would recite Umm al-Qur'an, the people would say Ameen, so Sayyiduna Umar (may Allah be pleased with him) would also say Ameen, and he considered it a Sunnah.
