Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 166

وَعَنْهُ قَالَ : { كَانَ رَسُولُ اَللَّهِ – صلى الله عليه وسلم -إِذَا فَرَغَ مِنْ قِرَاءَةِ أُمِّ اَلْقُرْآنِ رَفَعَ صَوْتَهُ وَقَالَ : “آمِينَ” . } رَوَاهُ اَلدَّارَقُطْنِيُّ وَحَسَّنَهُ , وَالْحَاكِمُ وَصَحَّحَهُ . 1 . 1 – صحيح بما بعده ، رواه الدارقطني (1/335) ، والحاكم (1/223) .

Narrated [Abu Hurairah (RA)]: When Allah’s Messenger (RA) completed the recitation of Surat Al-Fatiha, he raised his voice and said Amin. [Reported by ad-Daraqutni who graded it Hasan (good) and Al-Hakim who graded it Sahih (authentic)).