Gandalf says it with regard to Bilbo’s share of the troll treasure, a nice homey aphorism for coming home.  Our good friends at the OED report that “alike” probably has multiple sources.  The Scandinavian and German roots of the word use prefix 3 (“on” and “onto”), and the other source is Middle and obsolete Modern English “ylike”, the “y-” prefix indicating completeness being reduced to “a-” prefix 2.

• 19.026 and share alike!

“a-, prefix2.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“alike, adv.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.