The following solution works works without employing a minipage:
\documentclass{beamer}% http://ctan.org/pkg/beamer
\newcommand{\btVFill}{\vskip0pt plus 1filll}
\begin{document}
\section{SCS}
\subsection{SCS}
\begin{frame}
\frametitle{Simple Client-Server}
\bigskip
\rule{150pt}{100pt}%\includegraphics[scale=0.3]{scs}
\btVFill
source of image :write this text at the bottom of the slide (not footnote)
\end{frame}
\end{document}

You can also equally distribute the available space by employing the \btVFill command multiple times; the bottom line can be lifted a bit by adding some vertical space (such as \smallskip) after it:
\documentclass{beamer}% http://ctan.org/pkg/beamer
\newcommand{\btVFill}{\vskip0pt plus 1filll}
\begin{document}
\section{SCS}
\subsection{SCS}
\begin{frame}
\frametitle{Simple Client-Server}
\bigskip
\rule{150pt}{100pt}%\includegraphics[scale=0.3]{scs}
\btVFill
a comment in the middle
\btVFill
source of image :write this text at the bottom of the slide (not footnote)
\smallskip
\end{frame}
\end{document}

btVFillcommand? – Dr. Manuel Kuehner Oct 04 '13 at 13:14\vskip0pt plus 1filllis TeX syntax to insert a stretchable vertical skip with a minimum of 0pt, but stretchable to "infinite value". So TeX will stretch the vertical skip as much as possible (depending on the following content), which here gives the intended result to skip to the bottom of the slide. The details of stretchable skips are pretty well explained in this answer from JLDiaz. – Daniel Oct 06 '13 at 21:43\pausestatements in the frame have been processed, so only at the last slide of the frame. How do I make this text visible on all slides of the frame? – StrawberryFieldsForever Oct 22 '17 at 21:02\pause. In that case you should use\onslide<1->{bottom text}and replace all\pausecommands with the correct\onslidecommands. – hife Mar 08 '22 at 10:01