know from memory
know (something) from memory
To be able to recite or recount the entirety of something from memory alone, without having to consult some reference. OK, everyone, please keep practicing. I want you all to know this song from memory on Monday, all right? Knowing what to do from memory alone is not going to be helpful when you're trying to apply it in the field.
know something from memory
to have memorized something so that one does not have to consult a written version; to know something well from seeing it very often. (Almost the same as know something by heart.) Mary didn't need the script because she knew the play from memory. The conductor went through the entire concert without sheet music. He knew it from memory.