(a) Wa'il bin Hujr (may Allah be pleased with him) said: I saw the Messenger of Allah (peace and blessings be upon him); when he (peace and blessings be upon him) said: 'Not of those who have earned (Your) anger, nor of those who go astray' [Surah al-Fatihah: 7], he said: "Amin," and he drew out his voice with the Amin.
(b) Likewise Waki' bin al-Jarrah narrated it from Sufyan with these words: "He would draw out his voice in saying Amin," and al-Firyabi narrates from Sufyan in this hadith that he (peace and blessings be upon him) raised his voice when saying Amin and also prolonged it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2446