\documentclass{article}

\usepackage[latin2]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[magyar]{babel}
\newtheorem{tetel}{tétel}

\begin{document}

\begin{tetel}[{\cite
  [Pythagoras tétele]{geomk}}]
  $a^2+\allowbreak b^2= c^2$.
\end{tetel}

\end{document}