Q. If one hears someone saying a bracha, is there an obligation to answer Amen or is it only praiseworthy to do so?
A. The Beis Yosef (OC 215) writes that this is a matter of dispute between the Rambam and the Tur. The Rambam (Hilchos Berachos 1:13) writes that if one hears a Jew recite a bracha, even if he did not hear the entire bracha and even if he himself was not obligated in this bracha, nevertheless, he is obligated to answer Amen. However, the Beis Yosef understands the Tur (OC 215) to be saying that answering Amen to a bracha is not obligatory. The Bach and others interpret the words of the Tur differently, and they believe the Tur agrees with the Rambam. Regardless, the Shulchan Aruch (215:2) rules in accordance with the Rambam that if one hears a bracha, there is an obligation to answer Amen.
The Mishna Berura (215:8) writes that the obligation to respond Amen to a bracha is based on the Biblical verse, When I call the name of Hashem, bestow greatness to our G-d (Devarim 32:3).
Poskim write that one must be very careful to answer Amen to brachos. The Chayei Adam (6:1) and the Aishel Avrohom (OC 215) citing the Zohar Chadash (Rus) write that there is a severe punishment for those who do not answer Amen. On the other hand, Reb Yossi says that answering Amen is greater than reciting the bracha (Berachos 53b), and the Talmud relates that the gates of Gan Eden are opened for those who answer Amen with all their strength (Shabbos 119b).
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