Helen Keller wrote it in her biography of her first days at Radcliffe College, however she quoted a "wise Roman" from her readings. It probably was Cicero, or Ovid, but I cannot find out with certainty. I would bet on Cicero.
I believe the quotation has been attributed to several ancient Romans, including one called "Rumi". But, yes, Hellen Keller quoted it in her autobiography.
The line "To be banished from Rome is but to live outside of Rome" is from William Shakespeare's play "Julius Caesar." It is spoken by the character Brutus in Act III, Scene 2. If you want to find a specific line or quote from a play, a good starting point is to search for the title of the play along with the quote you are looking for. This can be done using a search engine or by referring to the specific edition or copy of the play where the line is likely to be found. In this case, you can search for "Julius Caesar" along with the quote "To be banished from Rome is but to live outside of Rome" to find the act and scene in which it appears.
