\documentclass{book}
\usepackage{xcolor}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{thmtools}
\usepackage{lipsum}

\colorlet{myred}{red!80!black}

\declaretheoremstyle[
spaceabove=\topsep, spacebelow=\topsep,
headfont=\normalfont\color{myred},
notefont=\mdseries\color{myred}, notebraces={(}{)},
bodyfont=\normalfont,
postheadspace=\newline,
headpunct=,
numberwithin=chapter,
postheadhook=\leavevmode%
  \interlinepenalty 10000%
  \vskip-1.3\baselineskip%
  \noindent{\color{myred}\rule{\textwidth}{1pt}}%
  \interlinepenalty 10000%
  \vskip0.3\baselineskip\noindent,
qed=\textcolor{myred}{$\blacksquare$}
]{mystyle}
\declaretheorem[style=mystyle]{example}

\renewcommand\theexample{\thechapter$.$\arabic{example}}
\renewcommand{example}{\rl{مثال}}

\usepackage{xepersian}
\settextfont[Scale=1.2]{Yas}
\begin{document}

\chapter{فصل اول}
این یک مثال است
\begin{example}
%\lipsum*[2]
\end{example}

\end{document}