1
general::
Phrase(s): 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.
McGrawhill's American Idioms And Phrasal Verbs