Rabbi Yitzchok Alfasi was known as the Rif. The Roshei Teivos of his name is רי"א, and as far as I know there is no other historic figure known as the רי"א. Then why was he known as the Rif?
It has become common practice to refer to most meforshim by acronyms of their name (eg Rashi, Rosh, etc.). When and why did this practice start, and is it proper?