\documentclass[a5paper]{article}

\usepackage[margin=2.5cm]{geometry}
\usepackage{parskip}
\usepackage{amsmath,amssymb,amsthm}
\usepackage{graphicx}
\usepackage[english]{babel}
\usepackage{hyperref}

\newtheorem{lemma}{Lemma}
\counterwithin{lemma}{section}

\geometry{
	paperwidth=9cm,
	paperheight=3cm,
	margin=0.0cm,
}

\begin{document}
	%\section{AA}
	\setcounter{section}{1}
	
	\setcounter{lemma}{8}
	
	\begin{lemma}
		Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor.
		
		\begin{proof}
			Aenean massa. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec quam felis, ultricies nec, pellentesque eu, pretium quis, sem. Nulla consequat massa quis enim.
		\end{proof}
	\end{lemma}
	
\end{document}