In some shuls it says on the ark, "Da Lifnei Mi Atah Omeid." Is there any source from which that comes?
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Its from Masheches Brachos 28b which brings a braisah which discusses a story when Rabbi Eliezer became sick his talmidim asked him how does one merit olam haba ,He answered a few things and ends of off with the phrase when you pray know who you are standing before.
Rashi explains: דעו לפני מי וכו' - כדי שתתפללו ביראה ובכוונה.
The words apparently appear in a number of places. This site sources them, in the plural, to the talmud, Brachot 28b.