Sayyiduna Wa'il bin Hujr (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) recited «وَلَاالضَّالِّينَ», he would say Ameen, and when saying it, he would raise his voice.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1281