Ibn Umm al-Husayn narrates from his mother, Sayyidah Umm al-Husayn (may Allah be pleased with her), that she prayed behind the Prophet (peace and blessings be upon him), and she heard him (peace and blessings be upon him) recite 'Master of the Day of Judgement' [Surah al-Fatihah: 4]; then, when he (peace and blessings be upon him) completed the recitation of 'nor of those who have gone astray' [Surah al-Fatihah: 7], he (peace and blessings be upon him) said 'Amin', so that she heard it clearly although she was in the women's row (at the back of the mosque).
In the chain of this hadith, Isma'il bin Muslim al-Makki is a weak narrator; however, the imam saying Amin aloud in the audible prayers is established by many other reliable and authentic hadith.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / صفة الصلاة / 477
Hadith Gradingمحدثین:إسناده ضعيف
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :إسناده ضعيف. » «أخرجه مجمع الزوائد ومنبع الفوائد برقم: 2669، وأخرجه شرح مشكل الآثار برقم: 5409، وأخرجه معرفة السنن والآثار برقم: 3178»