Fiqh-us-Sunnah Volume 001, Purification and Prayer, Fiqh 1.136.

Section : Sunnah acts of prayer, Saying 'Ameen.

It is sunnah for everyone to say 'ameen after reciting al-Fatihah. The word ameen is not part of al-Fatihah, but rather a supplication meaning, "O Allah, respond (to or answer what we have said). It should be said aloud in the prayers where the recital is aloud, and quietly in the prayers where the recital is silent. Said Na'eem al-Mujamir, "I prayed behind Abu Hurairah and he said, 'In the name of Allah, the Compassionate, the Merciful,' then recited al-Fatihah, and closed it with 'ameen. The people also said 'ameen. After the prayer, Abu Hurairah said, 'By the One in whose Hand is my soul, I have followed the prayer of the Prophet."

Al-Bukhari mentioned this hadith in mu'allaq from while others, such as an-Nasa'i, Ibn Khuzaimah, Ibn Hibban and Ibn as-Siraj related it. Al-Bukhari records that Ibn Shihab (az-Zuhri) said, "The Messenger of Allah, upon whom be peace, would say, 'ameen."

Says 'Ata, "'Ameen is a supplication." Ibn az-Zubair and those behind him would say 'ameen and the mosque would ring with their voices.

Reported Nafa', "Ibn 'Umar did not encourage the people to say it aloud, nor did he discourage them. I have heard him report that." Reporting on this same subject, Abu Hurairah said, "When the Messenger of Allah, upon whom be peace, would recite, '...Not with those with whom You are displeased and not of those who have gone astray,' he would say, 'ameen such that those close to him could hear him." (Related by Abu Dawud.) Ibn Majah's version is, "Until the people in the first row would hear him, and the mosque would ring with the sound." Al-Hakim also relates this hadith, and says that it is sahih according to the criterion of al-Bukhari and Muslim. Al-Baihaqi calls it hassan sahih. Ad-Daraqutni considers it as hassan.

A similar report from Wa'il ibn Jubair says, "I heard the Messenger of Allah, upon whom be peace, recite, '...and not of those who have gone astray,' and then say 'ameen, and make it long with his voice." This was related by Ahmad. Abu Dawud has it with the wording, "And he would raise his voice with it." At-Tirmizhi classifies it as hassan and states, "More than one knowledgeable companion and those who followed them have said that a person should raise his voice while saying 'ameen and not make it silent." Ibn Hajr holds that the chain of this hadith is sahih. Reported 'Ata, "I have found two hundred companions of the Prophet, upon whom be peace, in this mosque and when the imam recited,'...and not of those who have gone astray,' I heard them say 'ameen. "'Aishah reported that the Prophet, upon whom be peace, said, 'The Jews do not envy you for anything more than they envy you for the salutations and the saying of 'ameen behind the imam." (Related by Ahmad and Ibn Majah)

It is preferred to say 'ameen along with the imam, and not before or after him

Abu Hurairah reported that the Prophet, upon whom be peace, said, When the imam recites, '...not of those with whom You are angered nor of those who have gone astray,' you should say 'ameen. If this corresponds to when the angels say it, he will have all of his previous sins forgiven." (Related by al-Bukhari.) He also reported that the Prophet said, "When the imam recites, '...not of those with whom you are angered nor of those who have gone astray,' then say 'ameen (along with the imam), for the angels say 'ameen and the imam says 'ameen. If his 'ameen corresponds to the 'ameen of the angels, he will have his previous sins forgiven." (Related by Ahmad, Abu Dawud and an-Nasa'i.)