Siman 57

After Kaddish, the sheliach tzibbur says: "Barekhu et Hashem ha-mevorakh!" and the congregation answers: "Barukh Hashem ha-mevorakh le-olam va-ed!" and the sheliach tzibbur also repeats and says: "Barukh Hashem ha-mevorakh le-olam va-ed!" (And according to the Rambam in chapter twelve of Tefillah, he does not repeat and say "Barukh...". And such is the opinion of Maharam of Rothenburg.) in order to include himself in the blessing, for previously he said "Barekhu!", meaning: that the congregation should bless. Therefore, he also blesses the Holy One, blessed be He, as I have written below in siman 139. And one does not answer "Amen" after the sheliach tzibbur when he repeats and says "Barukh...". And there are those who answer, and they are mistaken. Our teacher, the Rema, wrote that it is the custom that the sheliach tzibbur elongates the "Barekhu," and the congregation says "Yitbarakh ve-yishtabach..." while he elongates the "Barekhu." Thus far his words. And this is when the sheliach tzibbur elongates with a melody. But at the time he says the words—one should say nothing. Therefore, we do not practice saying this, because among us he does not elongate with a melody during "Barekhu." And know that it is the custom to bow slightly during the saying of "Barekhu," but not like the bowings of Shemoneh Esrei. And it is proper. And even according to what will be explained in siman 139 that for the reading of the Torah one should not bow, nevertheless, at the time of prayer one bows. And such is the simple custom. And there is one who wrote not to turn his face from the east until he says "Barukh..." (Sha'arei Teshuvah). And I do not know the reason for this: for at the time of the reading of the Torah one does not turn the face to the east, and what is the difference between this and that? Rather, it is so automatically, for since he stands toward the east, why should he turn his face? But according to the law, it seems to me that there is no hindrance in this. (And what the Magen Avraham wrote in subsection 1 will be explained in siman 124 that it is not so, see there.)

It is forbidden to interrupt between Kaddish and "Barchu," and between "Barchu" and "Yotzer Or." And so too in Maariv. And even for a mitzvah matter it is forbidden to interrupt. And in a place where they were accustomed to delay the prayer for some matter – they should delay before Kaddish, as I wrote in siman 54. And even regarding this, our custom is not so, as I wrote there, see there.