latex text below arrow code example

Example: text over left right arrow latex

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\xLeftrightarrow}[2][]{\ext@arrow 0099\Leftrightarrowfill@{#1}{#2}}
\makeatother
\begin{document}
$A \xLeftrightarrow{\mathit{hello}} B$
\end{document}