My Rabbi said that it is a halachah to interrupt (most places) in prayer to say aleinu together with a minyon. What is the source for this?
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 with them (see also: http://halachipedia.com/index.php?title=Tachanun,_Ashrei,_Aleinu,_Shir_Shel_Yom ).