Which element is common in all acids?

All acids contain hydrogen, but not all hydrogen-containing compounds are acids. Most acids also contain oxygen.
http://science.howstuffworks.com/acid-info.htm/printable

If I might add to BRY2K's correct answer, the definition I had to learn at school was "an acid is a compound containing hydrogen replaceable by a metal". Thus HCl is an acid (hydrochloric acid) because the hydrogen can be replaced by a metal e.g. sodium giving NaCl, sodium chloride or common salt, whereas methane CH4 is not an acid since the hydrogen is not replaceable by a metal.

Because most acids contain oxygen, at one time it was thought that ALL acids contain oxygen. Indeed the very name "oxygen" comes from the Greek for "acid producer". The German word for oxygen is "sauerstoff" - "sour stuff" or "acid substance".