Hafiz Abdush Shakoor Tirmidhi
Benefits:
The first part of the mentioned narration is authentic, while its last part, in which an example is given, is weak. (Da‘if Targhib wa Tarhib: 269)
From the mentioned hadith, the virtue of saying "Ameen" in prayer is established. It is understood that one should say "Ameen" in prayer, and it is also understood that saying "Ameen" does not invalidate the prayer; rather, sins are forgiven. As for the view held by some people that saying "Ameen" invalidates the prayer, the mentioned hadith states that sins are forgiven. What is meant here are minor (saghirah) sins, because major (kabirah) sins are not forgiven without repentance, nor are the rights of people (huquq al-‘ibad) forgiven.
Hafiz Ibn Hajar rahimahullah says: What is meant is that all minor sins are forgiven, not major ones. For this reason, in (مِنْ ذَنْبِهٖ) the word (مِنْ) is for partition (tab‘idiyyah), not explanatory (bayaniyyah). Therefore, the translation of the hadith will be that some of his sins are forgiven. (Fath al-Bari: 2/265)
What does it mean for the "Ameen" of the inhabitants of the earth to coincide with the "Ameen" of the inhabitants of the heavens? The scholars have explained this in various ways. It refers to concurrence in time, meaning that when the angels say "Ameen," the worshipper should also say "Ameen" at that very moment.
Hafiz Ibn Hajar rahimahullah has quoted the statement of ‘Allamah Ibn al-Munir rahimahullah, the gist of which is as follows: In reality, the concurrence of the servants with the angels in word and time is for the wisdom that the followers (muqtadis) may be kept alert so that they do not become heedless in prayer, because the angels are not heedless; they say "Ameen" at the proper time, so the "Ameen" of the one who is alert will coincide with theirs. (Fath al-Bari: 2/265)
What is the meaning of "Ameen"? Hafiz Ibn Hajar rahimahullah says: The word (Ameen) is, like the word (صَہٍ), among the nouns of actions (asma’ al-af‘al), and according to the majority of scholars, its meaning is: O Allah, accept (our supplication). There are some other opinions regarding the meaning of (Ameen), but the sum of all is this very meaning. (Fath al-Bari: 2/262)
Source: Musnad Ishaq bin Rahwayh, Page: 159