The Ecole Glossary - Brief profile of Pope St. Sixtus III mentions that he founded the first known monastery in Rome. - http://www2.evansville.edu/ecoleweb/glossary/sixtusiii.html
Catholic Encyclopedia - Pope St. Sixtus III, also spelled Xystus, d. 440. - http://www.newadvent.org/cathen/14032a.htm
Popes Through the Ages - Account of the pontificate of St. Sixtus III, from the book by Joseph Brusher. - http://www.cfpeople.org/Books/Pope/POPEp44.htm