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I assume he means places where one is allowed to interrupt anyway (i.e., not in the middle of a bracha, pesukei dzimra, birchat shma, shemoneh esrei, etc.) in which case his source is presumably e.g. the Mishna Brurah 65:9 or Yalkut Yosef (Tefillah vol 2, p. 462) where they rule that when the congregation is saying "Aleinu...", one should also say it along ...
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