Hilchot Berachot 1:17-18 (Answering amen to your own beracha – part 2)
12:10 pm
Why do we answer amen to our own beracha of Boneh Yerushalayim towards the end of Birkat HaMazon, when there is another beracha afterwards? Why do we not answer amen at the end of the second beracha before Shma?